Dissident church leaders called for a boycott of next month's municipal elections in brazen defiance of security laws Sunday and demanded the government negotiate with Nelson Mandela on an end to apartheid.

Winnie Mandela, the wife of the detained black nationalist, endorsed the church leaders' stand that her husband's freedom would be meaningless without such negotiations and appealed for unity among the nation's 26 million black majority against minority rule."A friend of mine once said `apartheid is too strong to be fought by a divided nation,' " said Mrs. Mandela, making her first comments since her husband's hospitalization last month for treatment of tuberculosis fueled rumors of his release.

"We would like to emphasize that we should close ranks amongst us," she said. "We look forward to the day when we shall sit at a negotiating table in a climate that will have been made possible by your efforts."

Mrs. Mandela spoke before a crowd of about 300 people, including a number of foreign diplomats, at a gathering publicizing a new biography about her husband's role in the outlawed African National Congress before his incarceration in 1962.

She praised the West German Embassy, which provided a $15,000 grant to finance publication of the biography "Higher Than Hope," for having "stood by us . . . through very difficult times."

Author Fatima Meer said the publisher had printed a limited run of 2,000 copies, fearing authorities would ban distribution in South Africa. But she said the failure of police to intervene Sunday indicated it would be permitted for sale locally.

Nelson Mandela, who celebrated his 70th birthday July 18, was rushed from Pollsmoor Prison in Cape Town Aug. 12 to a hospital and shifted last week to a private clinic, suggesting the government had decided on a step-by-step release program to deflate public celebrations.

In Cape Town Sunday, Anglican Archbishop Desmond Tutu delivered a strongly worded sermon calling on both black and white voters to boycott the Oct. 26 elections, the first town council polls since bloody rioting swept black townships between 1984 and 1986.

He acknowledged security laws impose tough penalties for campaigning for a boycott, but said, "I am not defying the government, I am obeying God."

Brig. Leon Mellet, spokesman for the Law and Order Ministry, said efforts were under way to secure a text of Tutu's speech.