JOHN Kani was preparing to open his new play in Johannesburg. It was the day of the dress rehearsal, so he was busy and didn't really want to take the phone call. Eventually, after 15 minutes, his assistant told him: "You'd better come to the phone".

What he heard was someone asking him to hold for their boss. "I was ready to tell the boss to piss off," he recalls. "Then a voice said, 'it's Nelson Mandela'."

The African leader explained he was going overseas the next day and had been told by the Chief Justice that he must see Kani's play, Nothing But The Truth. So, he asked Kani if he could come and see it that night.

"It was almost like a royal command performance," says Kani. "So we performed the play and, at the end, he said, 'what a powerful family drama, John'.

"I was very proud. The play is political but in a very subtle way. It focuses on a family, although the backdrop is about truth and reconciliation."

Actor, director and playwright Kani has been combining politics and theatre for nearly half a century as the fight for freedom and democracy took place in South Africa. That's been followed by truth and reconciliation, a phrase he uses several times when we meet at Northern Stage, in Newcastle, where Nothing But The Truth opened a British tour.

The play is a tribute to his younger brother, Xolile, who was shot dead by police while reciting a poem at a 1985 New Brighton funeral and mass rally for a nine-year-old girl who'd died after being hit by a tear-gas canister.

He recounts the story of his brother's death, the horror all the greater for the calm way he relates the details, ending with: "Four bodies lay on the ground. One was my brother".

Kani has had his own brushes with the South African security police during the struggles. He was arrested, imprisoned in solitary confinement and interrogated. Called "the most dangerous protagonist of integrated culture", he was placed on a death list by extreme right-wing groups. He survived an assassination attempt in which he was stabbed 11 times.

Now 64, he's collected international humanitarian awards to place alongside the Olivier, the Emmy and other theatrical prizes he's won.

Sizwe Banze Is Dead, co-authored with Athol Fugard and Winston Ntshona, changed the face of theatre in South Africa. Now comes the very personal Nothing But The Truth.

Kani envisaged doing just a handful of performances when it opened five years ago. "Then I thought I'd get on with my life and this little story, which in a very strange way was a selfish one, would have been told," he explains.

He plays an ageing librarian who, along with his daughter and niece, is forced to face the past, both political and personal, after the death of his estranged brother.

Kani had never forgotten, or got over, the death of his own brother. "Every time I thought about him, there was a lump in my throat of pain, anger and bitterness, and a sense of being robbed of justice," he says.

"In 2001, I thought, 'isn't it time to deal with this? I started by writing a few lines to my brother explaining what the status is in South Africa today. Those two pages were the worst things I've ever seen.

"A month or two later I went back and the story took a very pleasant turn. I had this image of two brothers - one who stayed at home and worked, and the other a political activist who went into exile in England.

"It made me realise how much reconciliation is needed to take place among people themselves on a day-to-day basis at grass roots level. We had done so many bad things to each other in the name of the struggle."

Performing the play has been cathartic. Not least looking out at an audience of black and white, young and old, all openly sobbing. "It was like the air went through a passage into my brain and I just smiled," he says. "I went straight to my brother's grave and asked what had happened, and I walked away with a strange smile. I understood.

"Now I remember him with great pride. It's the moments that we spent together that are in the forefront, and the wonderfully stupid things he used to ask me and make me laugh. I remember with great pride he paid that price for me to be a free citizen of South Africa and the world."

Forty years ago young Kani was set to fight for the cause, left behind after failing to outwit his father's plan to stop him leaving home. Then he heard about a group of actors near his township home in New Brighton, Port Elizabeth.

"They were discussing a play called Antigone by Sophicles and whether Antigone was right to defy the law of the state. And the answer was yes, if the law is unjust. I was back in my underground political meetings.

"Immediately, I had a sense I could use this play, this art, to further my aims for the struggle of liberation. Maybe this could replace an AK-47."

Despite problems getting police permits, this feeling was reinforced performing for middle class white people in the local Shakespeare Reading Society. "It dawned on me that theatre is a powerful weapon for change. From that day, I chose work I believed contributed to the betterment of my people and advancement of my struggle."

His latest "weapon", Nothing But The Truth, has not only toured to the US and now the UK, but also been filmed and is a prescribed text in schools.

"All this is in tribute to my brother and all the young people who lost their lives during the struggle for liberation," he says.

He believes South Africa is on the right track, but that it faces a huge task in reconstructing life, with serious challenges in such areas as crime, Aids, environment and education. He sees the need to work with the working class movement in understanding the need for economic growth.

"This might sound a very political objective but, as an artist, I believe I can make a contribution to making people aware of those priorities through my work."

n Nothing But The Truth: West Yorkshire Playhouse, Leeds, from May 9-12. Tickets 0113-213770