NEW YORK — Ahmed Kathrada, who spent 26 years in prison, many of them alongside his close friend Nelson Mandela, for resisting the apartheid system of white minority rule in South Africa, died on Tuesday in Johannesburg. He was 87.

He had been hospitalized this month with a blood clot in his brain.

The office of South Africa President Jacob Zuma called Mr. Kathrada a “stalwart of the liberation struggle for a free and democratic South Africa.’’

Born to an Indian Muslim family, Mr. Kathrada was the most prominent Asian South African in the movement to end apartheid, the system of racial segregation and white domination.

Active in leftist politics since his teenage years, he came to prominence in July 1963, when he was arrested with other anti-apartheid activists in Rivonia, a northern suburb of Johannesburg where the South African Communist Party and the armed wing of the outlawed African National Congress had purchased an isolated farm to use as a meeting place. Among the others arrested was Walter Sisulu, secretary general of the ANC.

That October, Mr. Kathrada was indicted on charges of trying to overthrow the government, start a guerrilla war, and open the door to invasion by foreign powers. Sisulu was also indicted, as was Mandela, who had been in prison since 1962 but who faced new charges after authorities found documents at the Rivonia farm linking him to the ANC’s armed wing.

The Rivonia trial, which began in April 1964, became a signature moment in the struggle against apartheid. A high point came when Mandela, in a three-hour speech, told the judge that he was “prepared to die’’ for “the ideal of a democratic and free society in which all persons live together in harmony and with equal opportunities.’’

Eight defendants — including Mandela, Sisulu, and Kathrada — were convicted on June 11, 1964, of plotting a “violent revolution.’’ They were sentenced to life in prison, at hard labor.

Mr. Kathrada spent 26 years and three months behind bars, 18 of them on Robben Island, the apartheid regime’s most notorious prison.

Confinement was something of an education: He and his fellow prisoners deepened their conviction that only continued pressure, at home and abroad, would help end apartheid.

“It really confirmed our belief that the South African authorities do not suddenly undergo a change of heart,’’ Mr. Kathrada said in 1989.

He and his compatriots had suspected that they would be arrested, he said, and had prepared psychologically. They understood, he said, that the isolation of Robben Island — in cold, shark-infested Atlantic waters off Cape Town — was intended to break them.

“From the security police to the prison authorities, they tried to instill into our minds that we would be forgotten in a few years’ time,’’ Mr. Kathrada said. “They did everything to crush our morale.’’

For the first six months, he said, the prisoners were put to work breaking stones with hammers. Then they were sent to work in the prison’s lime quarry for more than a decade. At one point, Mr. Kathrada said, Mandela and Sisulu were put on a meager ration of rice gruel as punishment for supposedly not working hard enough.

Mr. Kathrada said that on arriving at the prison he and the mixed-race convicts were issued long trousers, while black convicts such as Mandela and Sisulu had to wear shorts without socks. Even sugar, coffee, soup, and other foods were apportioned to inmates according to lines of racial hierarchy.

Mixed-race convicts were also spared the brutality that was inflicted on less prominent prisoners, Mr. Kathrada said, though they were hardly exempt from mistreatment.

But any attempt to humiliate them only stiffened their defiance, Mr. Kathrada said.

“Because we were so close to the oppressor, it helped to keep us united,’’ he said. They went on hunger strikes to force concessions.

In 1982, Mr. Kathrada, Mandela, Sisulu and two fellow activists were transferred to Pollsmoor Prison, in the Cape Town suburb of Tokai. While in prison, Mr. Kathrada obtained four university degrees, two in history and two in African politics.

He was 60 when he was freed, in October 1989.

He left no doubt that his dedication to the African National Congress had not waned. “We will carry out whatever the ANC wants us to do,’’ he said at the time.

Mr. Kathrada later became a member of Parliament, wrote several books, and gave tours of Robben Island, to Margaret Thatcher, Fidel Castro, Jane Fonda, Beyoncé and, twice, to Barack Obama.

Though Mr. Kathrada remained loyal to the ANC — he served on the party’s National Executive Committee and ran its public relations department — in recent years he criticized the scandal-plagued Zuma, who has been in office since 2009.

Last April, Kathrada called on Zuma to resign, after the country’s highest court found that the South African president had violated his oath of office by refusing to pay back public money spent on renovations to his rural home.

Mr. Kathrada, who once said that his being denied the ability to have children was “the greatest deprivation’’ he endured in prison, is survived by his longtime partner, Barbara Hogan, a white anti-apartheid activist who was sentenced to 10 years in prison in 1982 for treason. She became a government minister after the fall of apartheid in the early 1990s.