JOHANNESBURG, South Africa—President Robert Mugabe faced deeper international isolation Wednesday, with African states demanding that a discredited runoff election be postponed and anti-apartheid icon Nelson Mandela rebuking the Zimbabwe leader for the first time.

Tougher sanctions, sporting bans and economic boycotts could be next —and world support may build for opposition leader Morgan Tsvangirai, who called Wednesday for talks on power sharing.

Regional heads of state from southern Africa met in Swaziland and said Friday’s runoff should be postponed until conditions permitted a free and fair vote.

President Bush said the runoff election appears to be a “sham,” joining the international condemnation of Mugabe’s actions.

In London, Mandela made a carefully worded but pointed attack on Mugabe, saying there has been a “tragic failure of leadership in our neighboring Zimbabwe.” The speech, at a fundraiser that included Prime Minister Gordon Brown and former President Bill Clinton, was the first time the former South African president has spoken publicly about the political crisis in Zimbabwe. His words are devastating for Mugabe and will weaken his claim to be a champion of African interests.

Although out of office for nearly a decade, Mandela remains a commanding and respected figure. He uses his influence sparingly, and it is particularly rare for him to publicly differ with South Africa’s current president, Thabo Mbeki. South Africans and other Africans have been increasingly questioning Mbeki’s leadership on Zimbabwe, so Mandela’s brief but sharp comments will have particular resonance.

For Mugabe, they are a rebuke from a leader he sees as a fellow freedom fighter, and will be hard to dismiss or ridicule—so often Mugabe’s response to criticism.

Tsvangirai made the call for peacekeepers in a commentary published Wednesday in British newspaper The Guardian. Asked about it at a news conference later in Harare, Tsvangirai said: “What do you do when you don’t have guns and the people are being brutalized out there?”

He stressed he was not calling for military intervention.

Deploying peacekeepers requires an international consensus that can be hard to build, and efforts can be blocked by governments expected to host contingents. South African Deputy Foreign Minister Aziz Pahad noted that efforts to deploy an AU-U.N. peacekeeping force for Sudan’s Darfur have been stalled.

The international community, though, has been considering, and taking, other actions on Zimbabwe.

Queen Elizabeth II stripped Mugabe of his knighthood, acting on the advice of her Foreign Secretary David Miliband, who has pointed to widespread violence and intimidation of Zimbabwe’s opposition ahead of the runoff in which Mugabe is the only candidate. Scores of opposition activists, including high-ranking party members, have been attacked or killed since the first round of the election in March.

Mugabe was made an honorary knight in 1994, when he was considered an anti-colonial hero. The queen’s move put him in the company of the late Romanian leader Nicolae Ceausescu, who had his title taken away in 1989 at the height of his nation’s revolution.

Bush called upon the African Union to continue to highlight the “illegitimacy” of the elections and keep reminding the world that the process is “not free and it’s not fair.”

Bush said the people of Zimbabwe deserve better.

“The people there want to express themselves at the ballot box, yet the Mugabe government has refused to allow them to do so. This is not just and it is wrong,” he said.

Brown’s spokesman Michael Ellam said sanctions against the southern African country are under review, but said Britain wanted to guard against hurting the population.

“We are mindful of the humanitarian impact on Zimbabwe, but we would not want to see anything that would prop up the regime in any way,” Ellam said.

European Union leaders last week threatened Zimbabwe with more, unspecified sanctions. Brown said then that could include more targeted sanctions against members of Mugabe’s regime.

EU nations already have an arms embargo in place against Zimbabwe in addition to a suspension of development aid and an assets freeze and travel ban against Mugabe and more than 100 other top government officials.