Former president Nelson Mandela had many qualities other leaders should emulate, Archbishop Emeritus Desmond Tutu said in Johannesburg on Monday night.

Tutu recalled how Mandela's personal assistant once phoned him to ask for the contact details of a commissioner of the Truth and Reconciliation Commission (TRC).

Tutu had asked Mandela to appoint a judicial commission to look into allegations by someone seeking amnesty that the commissioner was implicated in a crime.

When Mandela's assistant phoned, Tutu gathered Mandela "wanted to put this man out of his misery".

He told the assistant that, as TRC chairman, he should be informed of the outcome first.

"A few minutes later, Madiba calls. 'You are right, I'm sorry,' (he said)." This was evidence of Mandela's humility.

"There are not many heads of state who so easily say 'I'm sorry'," Tutu said at the Nelson Mandela Centre of Memory.

A foreign reporter asked him whether Mandela might feel disappointed in the leadership of the African National Congress today.

"We have come here to honour Madiba... Come next year and ask me that question," he replied.

Tutu said it was important to keep Mandela's values alive.

"And I hope that not only we, but our leaders, emulate him."

Earlier, Tutu led a prayer service in honour of Mandela.

"The best tribute each one of us can give is to embrace the values he gave us," he said during the service.

"He may not be physically with us, but let us not make a mistake and think he is not with us."

Hundreds of people who had not been allowed into the venue, because there was not enough space, gathered at the barricades erected nearby to hear the service over speakers. Candles were lit and masses of flowers of all colours placed along the barricade on 12th Avenue, near Mandela's Houghton home, where he died on Thursday evening.