SOWETO, South Africa (AP) _ Nelson Mandela was banned, and could not stay to eat the wedding cake with his bride in 1958. She has kept it, crumbling at the edges and no longer white, as a reminder.

South Africa's most prominent black spent his 24th consecutive birthday in prison Friday and Winnie Mandela, who has been with her husband less than a year since their marriage, displayed the cake and spent the day fasting as she always does.

Mandela, 68, is a symbol for blacks of the struggle against apartheid and a focal point for the outside world of pressure against racial discrimination in South Africa. The white government considers him a violent revolutionary.

The African National Congress leader was arrested in August 1962 for incitement, and while serving a five-year term was convicted in 1964 of sabotage and conspiracy to overthrow the government. He was sentenced to life in prison.

Last year, President P.W. Botha offered to release Mandela if he renounced violence. Mandela refused, and said the government itself must renounce violence, dismantle apartheid and lift the ban on the ANC, which was outlawed in 1960 and now has its headquarters in Lusaka, Zambia.

Repeated rumors of an impending release have proved groundless.

''I'm afraid the key is in Pretoria's hands,'' Mrs. Mandela said Friday. ''I don't think any of us has any idea what the (government) is planning,'' She spoke to reporters on the lawn of her four-room home in Soweto, the black city of 2 million people outside Johannesburg.

Winifred Nomzamo Madikizela was 22 when she met Mandela in 1956. Their marriage two years later never was formally completed according to tribal tradition.

He was under a banning order at the time and was not allowed to be in the rural Transkei homeland for the full four-day ceremony, of which eating the wedding cake was to have been the final act.

The two-layer cake, which sat on a table on the lawn, has nearly fossilized in those 28 years.

''We have always symbolized this day, which would have been a family day in any normal, civilized society, by taking out this cake, which has seen a lot of days,'' Mrs. Mandela said. ''We have kept the cake because we have believed that there will be a day when he will come home and see what is left of it.''

After the marriage, Mandela was organizing, on the run, out of the country or in jail most of the time until his arrest. He was held on Robben Island off Cape Town until 1983, and since then has been in Pollsmoor Prison on the mainland, south of Cape Town.

Mrs. Mandela said the birthday ''isn't any different from the other birthdays throughout the years. We have regarded this day in the family as a day of private prayer for our country and a day of fasting.''

''We have had nothing to rejoice about,'' she said of herself and their two daughters. ''It always has been a reminder that we've never had any normal family life.''

She visited her husband twice in the past week. ''There has never been any change in his spirits,'' she said. ''He's always been the same man who went to prison in 1962. His spirits have never been daunted.''

Since his hospitalization in November for routine prostate surgery, he has been kept in the prison hospital, isolated from four other black leaders sentenced with him in 1964, Mrs. Mandela said.

''Now, there seems to be a hardening of attitudes within the prison administration. He has to submit an application if he wants to see Uncle Walter,'' she said, referring to his 74-year-old ANC colleague Walter Sisulu, who also is in Pollsmoor.

Mandela is ''delighted at the solidarity of the caring outside world'' in pressuring South Africa to end apartheid, his wife said.