Mangosuthu Buthelezi, the Zulu nationalist who positioned himself as Nelson Mandela’s most powerful Black rival in South Africa’s tortuous transformation from a white segregationist society to a multiracial democracy in the 1990s, died on Saturday. He was 95.

His death was announced in a statement by President Cyril Ramaphosa of South Africa. It did not say where he died or give a cause. He said Mr. Buthelezi had been “an outstanding leader in the political and cultural life of our nation, including the ebbs and flows of our liberation struggle, the transition which secured our freedom in 1994 and our democratic dispensation.”

In the political turmoil of apartheid’s final years, Mangosuthu Buthelezi (pronounced mahn-goh-SOO-TOO boo-teh-LAY-zee) was the third man in South Africa: the linchpin with whom F.W. de Klerk, president of the white minority government, and Mr. Mandela, a global symbol of resistance to injustice released from prison after 27 years, had to reckon to hammer out a new Constitution and the future of the nation.

Proud, ambitious, descended from royalty and intolerant of criticism, Mr. Buthelezi was a hereditary chief of the Zulus, South Africa’s largest ethnic group. Like his battle-hardened ancestors, who had challenged colonial invaders in the 19th century, Mr. Buthelezi sometimes wore leopard skins and wielded assegai spears, but only in ritual war dances for political advantage. He was also the prime minister of KwaZulu, the homeland of six million Zulus, and the founder of the Inkatha Freedom Party, a Zulu political and cultural movement with 1.9 million members.

Beyond claiming to speak for nearly a quarter of the nation’s 28 million Black inhabitants in 1990, Mr. Buthelezi appealed to many white South Africans by advocating a peaceful transition to democracy and free enterprise, and by fiercely opposing Mr. Mandela’s African National Congress and its demands for international sanctions, armed struggle and a socialist revolution to crush Pretoria’s segregationist regime.

But Mr. Buthelezi was an enigma.

To his many admirers, he was a statesman, posing in the Oval Office with Presidents Ronald Reagan and George H.W. Bush; having tea with Prime Minister Margaret Thatcher of Britain; visiting Pope Paul VI at the Vatican; and cultivating a persona as the man who might lead South Africa when white rule eventually gave way.

Historians and human rights activists, however, said that for decades, as Mr. Mandela languished as a prisoner of conscience, Mr. Buthelezi amplified his power with devious stratagems. Instead of challenging Pretoria, which might have landed him in prison, he opposed apartheid from within governing structures, critics said, rejecting nominal independence for KwaZulu and running his homeland like a dictatorship.

Controlling the police, the legislature, the courts and other levers of power, he repressed anti-apartheid groups with policies remarkably like those of Pretoria, critics said, ordering arrests, disrupting protests, dispensing patronage and denying jobs to dissenters. Many Black intellectuals and activists fled KwaZulu, the collection of 40 tribal homelands scattered across the former Natal Province on South Africa’s southeast salient. (After apartheid, KwaZulu became KwaZulu-Natal Province.)

Moreover, historians said, Mr. Buthelezi controlled Inkatha paramilitary fighters whose internecine clashes with A.N.C. militants claimed up to 20,000 lives in the late 1980s and ’90s. Besides financing the KwaZulu government, Pretoria admitted in 1991 that it had covertly subsidized Inkatha in its war with the A.N.C., reinforcing allegations that Mr. Buthelezi had collaborated with the white government.

“Depending on whom you talk to in South Africa, he is a tool of apartheid, a courageous opponent of white domination, a tribal warlord or a visionary proponent of democratic capitalism,” Michael Clough said in a review of Mr. Buthelezi’s book, “South Africa: My Vision of the Future” (1990), adding, “While he speaks eloquently of the need for nonviolence, his followers have been accused of murdering hundreds of their opponents in Natal Province.”

In 1990, when South Africa signaled its willingness to disband apartheid by freeing Mr. Mandela and lifting a 30-year ban on the A.N.C., Mr. de Klerk and Mr. Mandela became the principal negotiators for a new Constitution. But Mr. Buthelezi quickly inserted himself into the bargaining as a voice for capitalism, education, tribal and ethnic rights, and powers for regional governments.

Over the next few years, as debates at the table flared and factional fighting worsened, Mr. Buthelezi often boycotted the talks. But apartheid ended in hospitals, theaters, swimming pools, parks, libraries and public transportation. And a new Constitution emerged, creating a parliamentary democracy with executive, legislative and judicial branches, a Bill of Rights, a universal franchise and 10 regional governments.

“Thanks in large part to Mr. Buthelezi, the draft Constitution that will serve the country in its first years of liberty now guarantees provincial governments important powers, including the right to tax and control education and the police,” The New York Times reported.

In the first democratic elections in 1994, Mr. Buthelezi, after first balking, campaigned with gusto. But Inkatha won only 10 percent of the votes, and his hope for the presidency evaporated. Mr. Mandela became South Africa’s first Black president. He appointed Mr. de Klerk as deputy president and Mr. Buthelezi as minister of home affairs. (Mr. de Klerk resigned as deputy president in 1996.)

When Mr. Mandela left office in 1999, Mr. Buthelezi retained the home affairs portfolio under President Thabo Mbeki until 2004. He also held a seat in Parliament for two decades. In 2019, he stepped down as party leader to be replaced by Velenkosini Hlabisa, the party’s provincial party leader in the KwaZulu-Natal assembly. Mr. Buthelezi continued to head the party’s caucus in the national Parliament.

In the aftermath of apartheid, a Truth and Reconciliation Commission, which was set up to document abuses of the era, concluded in 1998 that Mr. Buthelezi had collaborated with the white regime, and that Inkatha had massacred thousands of opponents. Mr. Buthelezi challenged the findings in court, and, as part of a 2003 settlement, the authors dropped his name from the final report.

Mangosuthu Gatsha Buthelezi was born in Mahlabathini, South Africa, on Aug. 27, 1928, to Chief Mathole Buthelezi and Princess Magogo kaDinuzulu. His parents were Zulu royalty, descended on his mother’s side from King Cetshwayo, who inflicted a historic defeat on British forces at Isandlwana in 1879, and on his father’s side from Chief Mathole Buthelezi, the prime minister for King Solomon kaDinuzulu. Mr. Buthelezi’s mother was King Solomon’s sister.

He attended Adams College, near Durban, from 1944 to 1947, then the University of Fort Hare from 1948 to 1950. He was expelled for taking part in political protests after joining the African National Congress Youth League. He later finished college at the University of Natal.

In 1952, he married Irene Mzila, a nurse. They had three sons and five daughters, and were married almost 67 years before Princess Irene died in March 2019. Mr. Buthelezi is survived by three of their children — Prince Ntuthukoyezwe Zuzifa, Princess Phumzile Nokuphiwa and Princess Sibuyiselwe Angela.

He became chief of the Buthelezi clan in 1953 and of the Zulu Territorial Authority in 1970 and the KwaZulu Legislative Assembly in 1972. Fiercely anti-Communist, he broke with Mr. Mandela, an early friend, and severed ties with the A.N.C. when its anti-apartheid struggle turned violent. He argued that violence was immoral, that economic boycotts and international sanctions robbed Black people of jobs, and that free markets would foster prosperity.

In 1975, he revived the moribund cultural association Inkatha, and used it as a political power base to mobilize Zulu nationalist aspirations, capitalizing on images of fierce warriors like the renowned Shaka Zulu, who united a 19th-century Zulu kingdom with military conquests, diplomacy and patronage.

In 1976, after Pretoria designated 10 tribal homelands to enforce apartheid, Mr. Buthelezi was named prime minister of KwaZulu. But his appointment, reaffirmed by successive white governments, was triple edged. It split the Black opposition to white rule, reinforced perceptions that he was Pretoria’s stooge and made him more powerful than ever.

By 1990, when apartheid began to topple, Mr. Buthelezi was a formidable political force, speaking for nearly a quarter of the nation’s Black people — a statesman to some, a scoundrel to others, but one who could not be ignored at the negotiating table.

“Apartheid is doomed,” Mr. Buthelezi said in an interview with The Times. “No matter how you analyze the South African situation, status quo apartheid politics is a thing of the past. The State President, Mr. F.W. de Klerk, has burned his bridges behind him, and for him there is no going back.”

Alan Cowell contributed reporting.