By NORIMITSU ONISHI

LISBON — How the roles have reversed: The colonizer, some Portuguese contend, has been colonized.

On the Portuguese coast of Cascais, where the nation’s royal court used to summer, a new 14-story condominium building looms confidently by the sea. So many of its apartments have been bought by Angola’s ruling class — sometimes a handful at a time — that the development has a nickname: the “Angolans’ building.”

Along the grandest shopping boulevard in the capital, Lisbon, Angola’s elite buy designer suits and handbags by the armful. And on one corner, above Louis Vuitton, sits the local office of Africa’s richest woman, Isabel dos Santos, a billionaire from Angola who has become one of Portugal’s most powerful figures by buying large chunks of the country’s banking, media and energy industries.

The money flowing into Portugal comes from the colony it dominated, often brutally, for hundreds of years, Angola. Now, the African nation is a major oil producer that has been led for the last 38 years by Ms. dos Santos’s father, President José Eduardo dos Santos.

Angola’s ruling class has profited so much during his tenure — and channeled so much of that money into Portugal — that when Angola threatened to cut off ties in recent years in response to reports that Angolan officials were being investigated for corruption in Portugal, Portugal’s foreign minister promptly apologized, setting off an intercontinental debate about the changing power dynamics between the nations.

“We had it in our heads that Angola was a poor country that needed to be helped,” said Celso Felipe, a Portuguese journalist and author of the book “The Angolan Power in Portugal.”

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“And suddenly they were able to help us and to buy things that we cannot buy,” he said. “It was like a housekeeper buying your house. That is awkward.”

The conditions in both countries created a perfect match: As Portugal reeled from a financial crisis a few years ago, Angolans were enjoying an oil boom that provided enormous opportunities for self-enrichment by the elite, particularly the president’s family and inner circle.

Angola is often listed as one of the world’s most corrupt nations. And Portugal has been singled out for its laxness in reining in money laundering and bribery, particularly in its dealings with Angolans, according to the Organization for Economic Cooperation and Development, the research and policy organization of the world’s richest countries.

“In Angola, they call Portugal the laundromat,” said Ana Gomes, a Portuguese lawmaker in the European Parliament and a member of Portugal’s governing Socialist Party. “It’s because it is.”

But the two nations’ relationship has now slipped into a tense and fluid period. Oil prices are down and Portugal’s economy is reviving, leading to a tweaking in the balance of powers between the two countries. And Mr. dos Santos is due to step down after Angolans elect a new president on Wednesday, leaving the future of those who have benefited from his four decades in power — both in Angola and in Portugal — unclear.

In a case that has angered the Angolan government, Angola’s vice president, Manuel Vicente, was charged in February with paying a $810,000 bribe to a Portuguese judge to quash a corruption investigation, the furthest an Angola-related case has moved in Portugal’s judicial system. The vice president was accused of, among other things, laundering money by buying apartments in the “Angolans’ building” on the coast of Cascais. He has denied the allegations.

With billions invested in Portugal, including in some of its biggest public companies, the Angolans have bought Portuguese wineries, newspapers, sports teams and other trophies of the super rich. With Portugal giving them access to the rest of Europe and beyond, they have been catapulted, in a few short years, to the world’s jet set.

The Lisbon headquarters of NOS, the Portuguese telecom company in which Isabel dos Santos, the oldest child of Angola’s president, own shares. Credit Patricia de Melo Moreira for The New York Times
Projecting both glamour and gravitas, Ms. dos Santos, worth $3.5 billion according to Forbes, mingles with Hollywood and European celebrities, and recently gave a speech at the London School of Economics — all of which she has documented on her Instagram account.

In the past two years, her family has made a splash at the Cannes Film Festival: Ms. dos Santos sat in the front row of fashion shows, and Kim Kardashian cooed over a 404-carat diamond paraded at a party thrown last year by de Grisogono, the Swiss jeweler now owned by the dos Santos family.

“Thought I’d seen it all, this is the biggest diamond I’ve ever seen,” Ms. Kardashian tweeted.

Ms. dos Santos was upstaged this year at Cannes only by her younger half brother, Danilo dos Santos, who was recently granted major shares in a new Angolan bank, according to the Angolan news media as well as politicians and businessmen in Luanda, Angola’s capital. At a charity auction, Mr. dos Santos, who is in his mid-20s, made the winning bid of 500,000 euros, or about $590,000, for a collection of photographs.

In the next few hours, as video of the auction spread on social media, his spending caused widespread outrage in Angola, where poor health care contributed to a yellow fever outbreak that killed more than 350 people last year.

But in the moment, basking in the applause, Mr. dos Santos and a companion walked up to the stage and were hugged by the M.C., the star Will Smith, who shouted, “500,000 euros! Yes, yes, yes!”

Upmarket high-rise apartment buildings in Luanda, the Angolan capital. The oil boom disproportionately benefited the country’s ruling elite. Credit Joao Silva/The New York Times
Scrutiny Over Investments

Angola had been at war — fighting for independence from Portugal, which came in 1975, and then locked in a civil war — for four decades by the time peace finally arrived in 2002. Peace coincided with an extended oil boom that eventually propelled Angola, with only 25 million people, to become one of the top 20 oil producers in the world.

It was an extraordinary turnaround for a country that had been dominated for centuries by Portugal, which exploited Angola for the slave trade and its natural resources, before falling into a long and costly civil war fueled by the Cold War.

But the oil boom disproportionately benefited Angola’s governing elite, who moved enormous sums abroad. Between 2002 and 2015, Angolan companies and individuals poured $189 billion outside the country into often opaque investments, according to the Catholic University of Angola’s Center for Studies and Scientific Research in Luanda.

Inside Angola, one of the world’s most unequal societies, half of the working population lives on less than $3.10 a day.

Meanwhile, its former colonial ruler, Portugal, suffered from a financial crisis that forced it to obtain a $111 billion bailout from international creditors and downgraded its national debt, rated junk to this day. Portugal was desperate for investment.

António Monteiro, a former foreign minister of Portugal and the chairman of the country’s largest private bank, Millennium BCP, said that investments from Angola had helped many Portuguese companies survive, including his bank.

“It was an investor that was very welcome and, in certain moments, the only investor in Portugal,” he said.

The problem was how the money was obtained.

Politicians and businesspeople in both countries say that Angola is dominated by allies of the president with tentacles in every corner of the economy, allowing them to amass great fortunes in politically connected deals under mysterious circumstances.

As Angola’s ruling elite sought to safeguard its wealth outside their country — knowing that Mr. dos Santos’s rule would eventually end — Portugal’s political and business elite more than obliged.

A billboard in Luanda congratulating President José Eduardo dos Santos of Angola on receiving 99.6 percent of the vote last year in his party’s leadership contest. Credit Joao Silva/The New York Times
“If Angola was the front office of corruption, Portugal was the back office,” said João Batalha, the president of the Portuguese chapter of Transparency International.

In a scathing report, the O.E.C.D.’s working group on bribery said that Portugal’s “enforcement of the foreign bribery offense has been extremely low” — pointing out that cases involving Angola accounted for a third of all such bribery allegations against Portuguese firms. In a March report on money laundering and financial crimes, the United States State Department said, “Suspect funds from Angola are used to purchase Portuguese businesses and real estate.”

Portugal’s foreign minister, Augusto Santos Silva, said that the Portuguese judicial system investigated illicit investments without political interference. He said it had been a “mistake” for one of his predecessors, responding to news media reports that high-ranking Angolans were being investigated in Portugal, to visit Angola and apologetically take the Angolans’ side.

Portuguese business leaders say that Angolan investments unfairly attract the kind of scrutiny that money from elsewhere, including China, does not.

Luis Míra Amaral — who until last year was chief executive of Banco Bic Portugal, a bank whose biggest shareholder is Isabel dos Santos — said that in Angola and some other African states, the same individuals tended to be both political and business leaders.

“It’s not realistic that a small country like Portugal would be able to change that,” said Mr. Amaral, a former Portuguese minister of industry and energy.

Mr. Amaral said that all Angolans had to “justify the money” they put into Portuguese banks by showing assets in Angola, to help prevent money laundering.

“When I see Isabel dos Santos putting money into Portugal because she has a number of big companies in Angola,” he said, pointing to her big stake in Unitel, Angola’s biggest cellphone operator, “it is easy to justify.”

“After, you can put another question: how she was able to create this company,” Mr. Amaral added with a laugh. “It’s another question. It’s not my problem as a banking manager.”

On her Twitter account, Ms. dos Santos identifies herself with just one word: entrepreneur.

Helped by public relations officials in Lisbon, Ms. dos Santos has put forward an image of herself as a self-made businesswoman. In a 2013 interview, she told the Financial Times that she had had business sense from a young age and sold chicken eggs when she was just 6.

In a written reply to emailed questions, Ms. dos Santos told The New York Times that the egg anecdote was meant “to show that since a young age I had an entrepreneurial spirit.”

But in Angola, the story about the eggs fueled widespread ridicule and anger.

Asked whether it was possible that Ms. dos Santos’s fortune was self-made, Marcolino Moco, a former prime minister of Angola, said, “It’s possible to make us laugh. All her wealth comes from the fact that her father is the law.”

The oldest child of Angola’s president, Ms. dos Santos, 44, has gained large or controlling shares in Angola’s diamond, cellphone, banking and other industries over the years.

In her email, Ms. dos Santos said that, beginning in the early 1990s, she had started “a small beverage distribution and logistics business,” as well as a restaurant, an event production company and a walkie-talkie business.

Ms. dos Santos did not say where she had obtained the capital to invest in those businesses or in Unitel, which she now controls. She has also acquired big stakes in Portugal’s banks, the Portuguese energy giant Galp, the Portuguese telecommunications company NOS, and other businesses.

In 2015, Transparency International included Ms. dos Santos on a list of 15 cases that symbolized “grand corruption.” She responded by saying that her investments were “transparent.”

Last year, her father appointed her chief executive of Sonangol, the state oil company that the president has used over the decades to further political and business interests. A group of lawyers has tried unsuccessfully to remove Ms. dos Santos from her post, arguing that she, with no management track record or experience in the oil industry, was appointed by her father to erase evidence of her father’s embezzlement at Sonangol before he steps down.

The downtown Luanda headquarters of Sonangol, the Angolan state oil company, of which Ms. dos Santos is the chief executive. Credit Joao Silva/The New York Times
Rui Amendoeira, a partner at Vieira de Almeida, a Lisbon law firm hired by Ms. dos Santos to help run the oil company, said he had no comment on the case or on Ms. dos Santos.

Her appointment as head of the state oil company gave Ms. dos Santos another platform internationally. In March, she was a speaker at the annual CERAWeek energy conference in Houston. In April, she spoke about being an entrepreneur at the London Business School.

Ms. dos Santos’s critics say she has tried to gain respectability in the West by using her ill-gotten gains in Angola and laundering them in Portugal — an accusation she rejects.

“In my book respectability is not achieved by mingling with the ‘right people,’” she said in an email. “In my book respectability is something you gain and achieve through a lifetime of work, and actions worthy of esteem, coherent positive behavior, attitudes of good social standing and building a reputation with your friends and peers.”

The stadium of Sporting Lisbon soccer club, whose major shareholder is Álvaro Sobrinho, an Angolan businessman and former chief executive of Banco Espírito Santo Angola. Credit Patricia de Melo Moreira for The New York Times
A Continuing Inquiry

Respectability, some Angolans have found, can be acquired in Portugal — and lost.

Álvaro Sobrinho, the former chief executive of Banco Espírito Santo Angola, was born in Angola and now lives in Portugal, holding dual citizenship. In Portugal, he became the major shareholder in Sporting Lisbon, a major soccer team, and also bought two newspapers.

But he was eventually investigated by the Portuguese authorities for his role in the bank, the Angolan subsidiary of Portugal’s Banco Espírito Santo, which he headed for a decade until 2012. The bank was accused of misappropriating $5.7 billion by giving out loans — to the political elite and to Mr. Sobrinho himself — that were never repaid. Mr. Sobrinho has denied all accusations and has not been charged despite the long investigation.

But he is now regarded as a “thief” in Portugal, he said. He has sold off his newspapers. He said he had at least recovered his assets, which the Portuguese authorities had frozen for a few years. A court reaffirmed in May that the state cannot seize his assets, but prosecutors say that Mr. Sobrinho is still being investigated.

His assets include six apartments he owns with his family in Estoril Sol Residence in Cascais, the “Angolans’ building.” Brazilians, Russians and Portuguese, including some who made money doing business in Angola, also owned apartments but were never investigated for money laundering, he said.

The resentment, though, did not detract from his enjoyment of his apartments (“very beautiful, you can see the sea”) or of Cascais (“an amazing place”).

Correction: August 22, 2017
An earlier version of this article misstated when Kim Kardashian tweeted about a 404-carat diamond. It was in 2016, not “this year.”