Nelson Mandela, imprisoned leader of South Africa's black underground, says he is being harassed for rejecting an offer of conditional release made by President Pieter W. Botha three months ago, his wife, Winnie Mandela, said in a recent interview.

She said he told her during a recent visit in Cape Town's Pollsmoor Prison that he and four other leaders of the outlawed African National Congress imprisoned there have had their lives "made very difficult" since they rejected the offer.

"Nelson told me that they were being continually harassed in an indirect way," Winnie Mandela said in the interview, adding that she had been searched by prison guards before the visit for the first time in the 23 years that she has been visiting her husband in prison.

Asked for an official response to Winnie Mandela's allegations, a Prisons Department spokesman, Lt. Col. Danie Immelman, said Saturday: "Mandela's treatment does not differ from any other prisoner in the same category. . . . . The same applies to visitors to Mandela."

Reuter reported the following:

Nobel Peace Prize laureate Bishop Desmond Tutu called yesterday for black unity as members of two rival antiapartheid groups clashed in the coastal city of Port Elizabeth.

Witnesses said at least one member of the Azanian People's Organization was seriously injured and another apparently was abducted in the fighting with members of the United Democratic Front.

The incident took place against a background of continued rioting . Police said two men died in unrest at Duduza black township east of Johannesburg. One was shot to death Saturday by police and another was killed by a shopkeeper whose store was being attacked.