Jacques Vergès, the French lawyer who embraced anticolonial causes and the role of devil’s advocate on a world stage to defend war criminals, terrorists, dictators and other notorious villains of the 20th century, died on Thursday in Paris. He was 88.

The cause was a heart attack around 8 p.m. as he was preparing to dine with friends, according to his publisher, Éditions Pierre-Guillaume de Roux. He died in the Parisian house where the Enlightenment philosopher Voltaire once lived, the publisher said in a statement.

“The ideal place for the last theatrical act that was the death of this born actor who, like Voltaire, cultivated the art of permanent revolt and volte-face,” the statement said, reflecting the lawyer’s reputation for asking disturbing questions on behalf of notorious clients.

Is a killer a terrorist or a patriot? Can laws be used to judge good and evil? For more than 50 years Mr. Vergès (pronounced vehr-JEZ) raised such questions in defense of clients who claimed to be acting for political causes, although they were charged with genocide, crimes against humanity, bombings, hijackings and the murder of innocents.

They included the Nazi war criminal Klaus Barbie; the terrorist Ilich Ramírez Sánchez, a k a Carlos the Jackal; and Cambodia’s Khmer Rouge head of state, Khieu Samphan. Mr. Vergès also sought to defend the former presidents Saddam Hussein of Iraq, who was executed for crimes against humanity, and Slobodan Milosevic of Serbia, who represented himself in a war-crimes trial but died before a verdict.

Like many of his clients, Mr. Vergès, the son of a Vietnamese woman and a French diplomat, was an enigma. Assassins targeted him. There were hints of ties to secret services, to terrorists he defended and to Mao Zedong, Che Guevara and other revolutionaries. He was a confidant of Pol Pot, the tyrant blamed for the deaths of at least 1.7 million Cambodians. He married a terrorist he saved from the guillotine, but left her and his two children and disappeared for eight years.

“He’s a slippery man,” the director Barbet Schroeder, who made “Terror’s Advocate,” a 2007 documentary on Mr. Vergès and terrorism as a political weapon, told The New York Times in 2007. “You can never touch him. He loves the mystery. The reason is that there are certain things he cannot talk about. He would be in deep trouble if the truth came out.”

In a career that paralleled the postwar disintegration of colonial empires in Asia, Africa and the Middle East, Mr. Vergès rose to prominence in the late 1950s defending Algerians accused of terrorist bombings. Instead of contesting the evidence of French prosecutors in court, he insisted that the defendants were resistance fighters in a just war of liberation and challenged the legal and moral legitimacy of the trials.

While most of his clients were convicted, the trials drew international attention to Mr. Vergès, and long after Algerian independence in 1962 his tactics served as a blueprint for his cases, which became public platforms to indict France and other Western nations for what he called crimes of racist colonialism and the exploitation of third world peoples.

In the late 1960s, Mr. Vergès broadened his horizons, defending Palestinians charged with attacks on El Al aircraft in Athens and Zurich. He later represented members of the Red Army Faction in Germany, whose bombings he called the work of “soldiers in a noble cause.”

Mr. Vergès’s most famous case was his defense of Klaus Barbie, the wartime Gestapo leader known as “the Butcher of Lyon” for his role in the torture, execution and deportation to death camps of thousands of French citizens. After years in hiding, during which he is believed to have assisted Western intelligence services, Barbie was extradited from Bolivia to France in 1983.

At his war-crimes trial in 1987, Barbie walked out, refusing to hear testimony of his horrors. Mr. Vergès virtually ignored the charges, and attacked Israel, France and other nations for committing “crimes against humanity,” which he called “more serious” than those ascribed to Barbie. Critics said he had trivialized genocide to defend a monster. Barbie was convicted and sentenced to life in prison, where he died in 1991.

“I practice the ‘defense de la rupture,’ ” Mr. Vergès told The New York Times during his work on the Barbie case, referring to a tactic of confronting the judicial system rather than working within it. “My law is to be against all laws. My morality is to be against all morality.”

Jacques Vergès and his twin brother, Paul, were born on March 5, 1925, in Ubon Ratchathani, Siam, now Thailand. Their Vietnamese mother died when they were 3, and their father, Raymond Vergès, a French diplomat, raised them on the Indian Ocean island of Réunion. Paul became a founder of the Réunion Communist Party and a member of the European Parliament.

In 1942, with his father’s encouragement, Jacques sailed to England, joined Charles de Gaulle’s Free French Forces and fought in the Resistance. After the war, he studied law at the University of Paris, joined the Communist Party and, in 1949, became a leader of an anticolonial student movement. His student friends included Khieu Samphan and Saloth Sar, the future Pol Pot. In the early 1950s, Mr. Vergès led a Communist youth organization in Prague.

Returning to Paris, he became a lawyer in 1955, and gained fame defending Algerian guerrilla fighters. In a notorious case, Djamila Bouhired was convicted in 1957 of killing 11 people in an Algiers bombing and sentenced to death. As she awaited the guillotine, it was revealed that she had been tortured during questioning. Mr. Vergès campaigned for a reprieve. Many world leaders, including the Soviet leader, Nikita S. Khrushchev, demanded her release. Her execution was postponed, and in 1962 she was released.

Mr. Vergès and Ms. Bouhired were married in 1965. They had two children, Meriem and Liess. Information about his survivors was not immediately available.

The couple founded a popular magazine, Révolution, in 1968. In 1970, Mr. Vergès disappeared. His whereabouts remained a mystery, although he was rumored to be in Cambodia with Pol Pot and in the Middle East with Palestinian groups. He reappeared in Paris in 1978 and resumed his law practice.

His ties to Carlos the Jackal were murky, but probably dated to 1982, when he defended Magdalena Kopp, the terrorist’s girlfriend and accomplice (and later his wife), who was caught with explosives in Paris. Wanted for many terrorist acts in the name of Palestinian liberation in the 1970s and ’80s, Carlos, who was born in Venezuela, was captured by French agents in Sudan in 1994 and flown to Paris.

Mr. Vergès called it a kidnapping and represented Carlos in the early stages of his case, but they had a falling-out over tactics. After hiring other lawyers, Carlos was convicted in 1997 and sentenced to life in prison. The verdict and sentence were affirmed in another trial years later.

After Saddam Hussein was captured in 2003, Mr. Vergès, who had been hired to defend other ousted Iraqi leaders, offered to represent him, but the Hussein family chose another lawyer. Mr. Hussein was executed in 2006. Mr. Vergès also offered to defend Mr. Milosevic, but Mr. Milosevic chose to represent himself in a trial that began in The Hague in 2002. He died in 2006 before the case could be concluded.

In 2008, as Khieu Samphan made his first appearance before Cambodia’s genocide tribunal, Mr. Vergès, representing his old friend, created a tumultuous scene and stormed out after erupting at a panel of judges because documents for the pretrial hearing had not been translated into French.

He argued that his client had held no real power as Cambodians had died of starvation, disease, forced labor and massacres during the brutal Khmer Rouge drive to create a classless society. He insisted that the power — and responsibility for the Cambodian tragedy — had belonged to Pol Pot, who died in 1998.

Alan Cowell contributed reporting from London.