(comments from the speech, background on Zimbabwe)

By Andrew Quinn

JOHANNESBURG, May 31 (Reuters) - British Prime Minister Tony Blair urged rich nations on Thursday to keep their promises of financial aid to Africa, saying a failure to do so could threaten democracy and economic growth on the continent.

In a keynote speech in Johannesburg on the final leg of his farewell African tour as Britain's leader, Blair also said that African leaders must get tough on authoritarian governments, such as those in Sudan and Zimbabwe.

"Wealthy nations and Africa both face a choice ... Our challenge is to support the good. Africa's challenge is to eliminate the bad," Blair said in the speech.

"Next week at the G8 (Group of Eight) Summit leaders will show whether, having put Africa at the top of the global agenda, we have the perseverance and vision to see it through. I hope we have," the outgoing British leader said.

Blair's visit came on the eve of the G8 summit scheduled for Germany, during which Chancellor Angela Merkel has vowed to press rich nations to fulfil aid pledges to Africa under a 2005 Blair initiative.

Due to resign and hand over power to finance minister Gordon Brown on June 27, Blair is using the trip to build momentum for the summit, which will focus on the world's poorest continent and push for a world trade deal.

He visited Libya and Sierra Leone before travelling to South Africa, where he will bid goodbye to former South African President Nelson Mandela and meet Thabo Mbeki, the country's current leader.

Pushing the United States and other Western nations to meet their pledges of financial aid, trade support and assistance on peacekeeping and conflict resolution is a key part of the Blair agenda in his final weeks in office.

But Blair is also highlighting what he says is the need to increase pressure on Zimbabwe President Robert Mugabe's government, which has been criticised in the West for a violent crackdown on political opponents.

Mbeki is overseeing an effort to bring Mugabe and his opponents in Zimbabwe to the bargaining table ahead of elections in the southern African nation scheduled for next year.

Blair and Mbeki have taken starkly different approaches to the crisis, with Britain pushing for more public action and South Africa advocating a policy of "quiet diplomacy" in hopes of nudging Mugabe into making changes. (Additional reporting by Eric Onstad in Johannesburg)

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