JOHANNESBURG, South Africa -- The state prosecutor attacked Winnie Mandela yesterday for waiting two years to tell authorities that she was out of town when four young men were allegedly abducted and beaten at her house in the black township of Soweto.

Prosecutor Jan Swanepoel questioned Mrs. Mandela as she took the witness stand for the second day. He raised repeated doubts about the alibi that she offered in testimony the previous day -- that she had been nearly 200 miles from home on the night of the incident.

Under tough questioning, Mrs. Mandela admitted that she had never told the police or the attorney general that she was in the town of Brandfort on Dec. 29, 1988, and did not return to Soweto until two days later. She agreed the claim had never been made until her trial began in February, when her lawyer read a statement to the court contending that Mrs. Mandela knew nothing of a kidnapping or an assault.

Mrs. Mandela said she was acting on her lawyer's advice in not revealing the information sooner. She rejected the prosecutor's suggestion that she could have easily cleared her name by coming forward with her alibi right away.

Mrs. Mandela, 56, the wife of African National Congress leader Nelson Mandela, is on trial along with two others on charges of kidnapping and assault. Two young men have testified that they were forcibly taken from a Methodist Church home and held against their will at Mrs. Mandela's home. They said they were beaten by several people, including Mrs. Mandela herself.

But the anti-apartheid activist contended during testimony yesterday that police fabricated evidence against her in order to damage her reputation.

"There is clear evidence the interest of the police was to bring us into this court," she told Mr. Swanepoel.

Mr. Mandela accompanied his wife to court and listened to testimony for a half-hour before leaving.