LONDON (AP) _ Nelson Mandela, jailed leader of South Africa's black independence movement, said in an unprecedented interview that his men would lay down their arms if South Africa agreed to negotiate with his banned African National Congress.

Mandela gave the interview to Lord Bethell, a writer and Conservative member of the European Parliament, for the London newspaper Mail on Sunday. Bethell said it was the first time anyone ''from the outside world has been able to see him or talk to him'' since he was jailed in 1964.

Sen. Edward Kennedy, D-Mass., was barred from visiting Mandela while in South Africa earlier this month.

Bethell received permission to interview Mandela at a time President P.W. Botha was announcing a seemingly more conciliatory approach by his white- minority government to the nation's black majority. Bethell did not say on what day the interview was held, but wrote, ''Last week I met him, and am able to set the record straight.''

Mandela, jailed for life in 1964 after being convicted of plotting sabotage and advocating the overthrow of the government and its policy of racial separation, appeared fit, self-assured and had no serious complaints about conditions at Pollsmoor Prison in Cape Town, Bethell wrote.

But he quoted Mandela as saying his first 10 years of imprisonment on Robben Island off the Cape Town shore ''were really very bad. We were physically assaulted. We were subjected to psychological persecution'' and had to work for nine hours a day in a lime quarry.

Mandela, 66, also was quoted as saying:

''The armed struggle was forced on us by the government, and if they want us to give it up, the ball is in their court. They must legalize us, treat us like a political party and negotiate with us. Until they do, we will have to live with the armed struggle.

''It is useless simply to carry on talking. The government has tightened the screws too far. Of course, if there were to be talks along these lines, we in the ANC would declare a truce.''

Meanwhile, he said, his fighters would strike only at military targets and buildings and property that serve apartheid - racial separation. He said he regretted any loss of innocent lives and called the May 1983 bombing in Pretoria that killed 17 civilians ''a tragic accident.''

He said ''I appreciate the Soviet Union'' because it had been an early supporter of black independence campaigns, but ''it does not mean I approve of their internal policy.''

Bethell said he spent two hours with Mandela in the presence of a prison warden, Maj. Fritz Van Sittert, who never spoke during the interview. He said Mandela spoke well of Van Sittert and his other jailers, and said reports of his health deteriorating were incorrect.

''I am in good health. It is not true that I have cancer. It is not true that I had a toe amputated.... Things get exaggerated because of lack of communication,'' he said.

Mandela told Bethell he exercises daily and tends a vegetable garden. He shares a cell with five other blacks convicted of offenses against the state. He reads South African, British and American newspapers and listens to local radio broadcasts.

Mandela said he is allowed one 45-minute visit a month by his wife, Winnie. Only last year did authorities allow these visits to include physical contact, and only in the presence of a guard, Bethell wrote.

Bethell quoted Mandela as saying he ''completely rejected the idea,'' that he leave prison and live in one of the black homelands established by South African.

''My place is in South Africa and my home is in Johannesburg,'' Mandela said. ''If I were released I would never obey any restriction.''