JOHANNESBURG, South Africa — All Africans must speak out about injustices in places like Zimbabwe, Liberia's leader said Saturday during a speech honoring former South African President Nelson Mandela.

Ellen Johnson Sirleaf devoted her speech, a week before Mandela's 90th birthday, to painting an optimistic picture of Africa's future.

But the Liberian president said she could not ignore current troubles, and that it was her duty to "express my solidarity with the people of Zimbabwe, as they search for solutions to the crisis in their country."

The remark earned applause from Mandela and a crowd of several hundred gathered in a community hall in Soweto, the famed Johannesburg township.

Sirleaf acknowledged that Liberia was far from southern Africa, and did not share this region's history of British colonial rule.

"But I am, I hope, part of the new Africa; an Africa rooted in many of the values demonstrated by you, President Mandela," she said. "In that Africa, all Africans have a responsibility for our collective future. It is, therefore, my and our responsibility to speak out against injustice everywhere."

She offered her own country in West Africa as a cautionary example.

"In 1985, Liberia held a sham election that was endorsed by Africa and the world," she said. "Thirty years of civil war and devastation followed, with thousands dead and millions displaced. It need not have happened."

Sirleaf was among the few voices at a recent African Union summit denouncing a June 27 presidential runoff in Zimbabwe that followed months of brutal attacks on opposition supporters. Zimbabwean opposition leader Morgan Tsvangirai withdrew from the runoff because of the violence, but Zimbabwean President Robert Mugabe went ahead, and claimed overwhelming victory.

Sirleaf also had come out in support of U.N. sanctions the United States had proposed in part to force Mugabe to negotiate a power-sharing agreement with Tsvangirai. The sanctions were vetoed by Russia and China when put to a vote Friday before the U.N. Security Council. Sirleaf did not comment Saturday on the U.N. vote.

Sirleaf is the first woman elected president on the continent. The 69-year-old former World Bank official also is widely hailed as among a new crop of African leaders committed to democratic and economic reform.

Mandela, who addressed the crowd only briefly, called Sirleaf "an inspiring example to Africa and the world." He also joked that the annual lecture — in the past given by Nobel peace laureates Kofi Annan, Wangari Maathai and Desmond Tutu, as well as former President Clinton and current South African President Thabo Mbeki — drew luminaries "principally to see what an old man looks like."

Sirleaf titled Saturday's speech "Behold the New Africa," and said that despite setbacks in Zimbabwe and elsewhere, she believed the continent was overcoming dictatorship and poverty. She cited economic growth averaging 5 percent in recent years, the relief of the foreign debt burden many countries had faced, and political change.

"It is hard to predict the future and the change will not be easy or smooth in every country," she said. "But never before in world history have so many low income countries become democracies in so short a period of time."

She put the burden for continued reform on Africans themselves, citing fighting corruption and mismanagement as key.

The modern community hall where Sirleaf and Mandela spoke Saturday was built on the site where white and black South Africans gathered in 1955 and, before apartheid police broke up the meeting, adopted the Freedom Charter, pledging to fight for multiracial democracy. Four decades later, post-apartheid South Africa borrowed the Freedom Charter's declaration that the country "belongs to all who live in it" for the preamble to a new constitution.

Sirleaf called the Freedom Charter "a bold development manifesto," and said she and others across the continent intent on reform had been inspired by Mandela and other South Africans.

Turning to Mandela, she said: "If someday I am remembered as one of the many dreamers who came in your wake who, unable to fill your shoes, walked in your shadow to build a new Africa, then I can think of no better place to be in history."