HARARE, Zimbabwe (CNN) -- Former South African President Nelson Mandela has highlighted what he called the "tragic failure of leadership in Zimbabwe."
Mandela issued the statement in a communique at his 90th birthday celebration in London as international pressure on President Robert Mugabe's regime mounted on the eve of a presidential runoff.
The country's Electoral Commission ruled Wednesday that Friday's presidential runoff will proceed as planned despite the withdrawal of opposition candidate Morgan Tsvangirai, according to the panel's chairman.
But the Southern African Development Community warned that the violence in Zimbabwe may "undermine the credibility and legitimacy" of the runoff, and the group urged Zimbabwean authorities to "consider" postponing it.
"In the light of the violence and the charged political atmosphere, the political and security situation appears not to be permissive for holding the runoff election in a manner that would be free and fair," the SADC troika said in a communique released after representatives from two members of the troika, Tanzania and Swaziland, met in Manzini, Swaziland.
The third member of the troika, Angola, did not attend the meeting.
"Holding the elections under the current circumstances may undermine the credibility and legitimacy of its outcome," the communique said.
It appealed to "the responsible authorities in Zimbabwe to consider postponing the election to a later date."
"You can't have free elections if a candidate is not allowed to campaign freely and his supporters aren't allowed to campaign without fear of intimidation," U.S. President George W. Bush said at a national security advisers meeting with the U.N. Security Council.
Despite growing international and regional condemnation, Zimbabwe appears determined to hold the vote.
"The commission met today to deliberate on the contents and letter [from] Tsvangirai," Election Commission Chairman George Chiweshe told reporters at a news conference.
"It was unanimously agreed that withdrawal was well out of time and, for that reason, the withdrawal was of no legal force."
A journalist in the capital of Harare reported Chiweshe's comments to CNN. The journalist is not being identified because of concern for the person's safety.
News of the election plans came as the international community stepped up pressure on the Zimbabwe government led by President Robert Mugabe.
Governments across the globe were also clamoring for Mugabe to end to the violence plaguing the African nation. Queen Elizabeth II stripped Mugabe of his knighthood Wednesday, which he was awarded in 1994, Buckingham Palace said. See pictures of the violence »
Also Wednesday, British Prime Minister Gordon Brown said that Britain is ready to propose "intensified" financial and travel sanctions against named members of the Zimbabwean regime and also will cut some sporting ties.
On Tuesday, the Movement for Democratic Change hand-delivered a letter signed by Tsvangirai to the Zimbabwe Electoral Commission, saying it will not participate in the runoff with Mugabe. Watch Tsvangirai's call for peacekeepers »
"The violence, intimidation, death, destruction of property is just too much for anyone to dream of a free and fair election, let alone expect our people to be able to freely and independently express ... themselves," Tsvangirai wrote.
Despite his withdrawal, Tsvangirai's name will remain on the ballots because they are already printed, the commission said.
At a news conference Wednesday, the MDC leader made four demands: Election-related violence must stop; humanitarian assistance must be allowed to operate freely; political prisoners must be released; and parliamentary legislators must be sworn in.
He called on international leaders to intervene, specifically the African Union and the Southern African Development Community, a regional body of 14 southern African nations.
Condemning the election as a "sham" and an "illegitimate exercise," Tsvangirai told CNN that Mugabe seems intent on holding a "one-man race."
"We will have nothing to do with a post-27th government coming out of these elections," Tsvangirai said during his Wednesday news conference. "What [Mugabe's party] ZANU-PF is trying to do is force an election on the people. It will be sad if they proceed on that basis." Watch Tsvangirai talk about Zimbabwe's desperate times »