Johannesburg - The leadership of former South African president Nelson Mandela and that of Zimbabean President Robert Mugabe were contrasted in a speech by Liberian President Ellen Johnson-Sirleaf in South Africa Saturday.

Addressing Mandela during the annual Nelson Mandela lecture in Johannesburg, Johson-Sirleaf said: "We admire you for returning justice and democracy to your country South Africa and in doing so becoming an aspiration for Africans and people the world over."

Mandela's efforts, had, among other things, led to free and fair elections in South Africa, the emancipation of women and a vibrant media, Africa's first elected female president praised.

The Nelson Mandela lecture, now in its sixth year, is delivered each year by a leading statesman or woman on the eve of the anti-apartheid icon's birthday.

After paying homage to Mandela, who turns 90 on July 18, Johnson-Sirleaf turned her attention to Zimbabwe, where 84-year-old President Robert Mugabe was sworn in last month for a controversial sixth term following elections he alone contested.

If the current political stalemate in Zimbabwe persisted, "the dream of a democratic and accountable government will remain unfulfilled," Johnson-Sirleaf said.

The Liberian leader warned Zimbabwe's southern African neighbours, including South Africa, they would continue to suffer the fallout of the instability in Zimbabwe unless tensions between Mugabe's Zanu-PF and the opposition Movement for Democratic Change were resolved.

Explaining her intervention, Johnson-Sirleaf said: "I am part of the new Africa and it is therefore my responsibility to speak out against injustice everywhere."

Johnson-Sirleaf is one of only a handful of African leaders to openly challenge the legitimacy of Mugabe's rule after he was sworn in as president for another five years in June.

Mandela also took to the podium with some advice for Africa and the world.

"If, at 90 years old, I offer some unsolicited advice on this occasion, it would be that you, irresponsible of your age, should place human solidarity, as a concern for the other, at the centre of values by which you live."

Earlier, a group of residents of Kliptown squatter camp, near the Soweto hotel where the lecture took place, handed in a letter of protest to the Nelson Mandela Foundation, saying Mandela's 1994 promise of a better life for all had yet to be fulfilled. (dpa)