Walter Sisulu, 90, the South African political activist who helped Nelson Mandela lead the successful struggle to end white minority rule in their country, died May 5, it was reported in Johannesburg.

The African National Congress, the political party that Mandela headed, said that Mr. Sisulu had been ill for a long time.

Often known as the elder statesman of the liberation movement, Mr. Sisulu won a reputation over the years as an organizer whose role was to plot strategy and conceive tactics.

But he was also physically in the thick of the struggle -- frequently harassed and arrested, and fearless about making his voice heard in the courtroom.

After being sentenced in the early 1950s for his role in a campaign to defy discriminatory laws, he told the court:

"I wish to make this solemn vow and in full appreciation of the consequences it entails.

"As long as I enjoy the confidence of my people, and as long as there is a spark of life and energy in me, I shall fight with courage and determination for the abolition of discriminatory laws and for the freedom of all South Africans irrespective of color or creed."

He and Mandela worked shoulder to shoulder for years in the long effort to build the ANC and to eliminate racial discrimination and separation in South Africa.

Mr. Sisulu was secretary general of the ANC in the 1940s and later became deputy president.

As a young man who had made a few connections in Johannesburg, Mr. Sisulu helped Mandela get a foothold there in the early 1940s.

Later in that decade, he joined with Mandela in revitalizing the ANC, setting it on a course of vigorous but nonviolent protests.

In the 1960s, Mr. Sisulu and Mr. Mandela were among antigovernment leaders who were placed on trial and convicted of treason in connection with their increasingly determined opposition to the official policy of apartheid, or racial separation.

They both were sentenced to life terms.

Mr. Sisulu was freed in 1989 after more than 25 years. In 1990, the government lifted a ban on the ANC, and in 1994, Mandela was inaugurated as South Africa's first black president.

"We walked side by side through the valley of death," Mandela said of Mr. Sisulu in a statement to the South African Press Association. "His absence has carved a void. A part of me is gone."

Mr. Sisulu, the son of an African domestic worker and a white civil servant, was brought up by his mother, an uncle and grandparents in the Transkei. His formal education ended in his mid-teens, but he worked to teach himself, and he was said to have been inspired from youth by the Bible story of David, the young shepherd who slew the giant Goliath.

According to students of his life, he resented from youth the deference paid to whites by his black neighbors and relatives. He went to work at laboring jobs and came into conflict with white bosses. While riding on a train, he fought with a white conductor who was harassing a black passenger, and he was jailed briefly.

Increasingly restive, he joined the ANC in 1940. The organization, founded in 1912 to champion black rights, had become feeble and impotent. With Mandela and others, Mr. Sisulu was credited with forming the ANC Youth League in 1944, bringing new life to the parent organization. In the 1950s, he made a trip to the Soviet Union and other East Bloc countries and was reportedly impressed by what he saw.

As protests in South Africa became increasingly militant, the ANC was banned, and the government accused some of its members of organizing for guerrilla warfare and sabotage.

This ultimately led to the 1960s treason trial at which Mandela and Mr. Sisulu were both defendants.

At his wedding, one of the guests told the bride that she must limit her expectations, saying: "You are marrying a man who is already married to the nation."

Survivors include Mr. Sisulu's wife, Albertina, and five children.

Walter Sisulu spent more than 25 years in prison on a treason charge.