The decision yesterday of Zimbabwe’s opposition leader to withdraw from Friday’s presidential runoff election will trigger new sanctions against President Mugabe’s regime.

The British foreign secretary, David Miliband, said his government would pursue “very strongly” a round of sanctions at the U.N. Security Council. “I think that is important. It’s also important that African leaders continue to make clear that a government which violates the constitution in Zimbabwe … cannot be held as the legitimate representative of the Zimbabwean people,” he told Sky News.

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Those words followed Morgan Tsvangirai’s announcement that he would pull out of a second round of elections against Mr. Mugabe, who has ruled Zimbabwe for 28 years.

“We can’t ask the people to cast their vote on June 27 when that vote will cost their lives,” Mr. Tsvangirai said. “We will no longer participate in this violent sham of an election.”

Mr. Tsvangirai, the leader of the opposition Movement for Democratic Change, was referring to a campaign of violent intimidation the Mugabe regime launched after Mr. Tsvangirai’s decisive victory in the first round of presidential voting, on March 29. The MDC leader has been detained several times since then, and his deputy, Tendai Biti, has been held on charges of treason since his return to Zimbabwe from South Africa June 12.

South Africa, whose president, Thabo Mbeki, is Mr. Mugabe’s last major southern African ally, snubbed America on Thursday by sending a low-level diplomat to a U.N. Security Council session called by Secretary of State Rice to address the recent violence in Zimbabwe.

The Bush administration’s response to yesterday’s events was tough. “The government of Zimbabwe and its thugs must stop the violence now,” a White House assistant press secretary, Carlton Carroll, said in a statement. “All parties should be able to participate in a legitimate election and not be subject to the intimidation and unlawful actions of the government, armed militias and so-called war veterans.”

The Zimbabwe government responded to Mr. Tsvangirai’s decision to pull out by predicting a win for Mr. Mugabe and his Zanu-PF Party. “It’s a sure case that President Mugabe will win resoundingly. Now this is what Tsvangirai has been advised: Rather than face humiliation and defeat of this magnitude, he has been advised not to stand for the runoff election, and this is unfortunate,” the information minister, Sikhanyiso Ndlovu, said.

A policy analyst and expert on southern Africa at the Cato Institute, Marian Tupy, said the opposition leader’s decision to withdraw from Friday’s vote erodes the “last remnants of legitimacy for the Mugabe regime.”

The move clears the way for an arms embargo against the embattled African country, Mr. Tupy said. “I think this is the best opportunity the international community has ever had to impose great sanctions on Zimbabwe and have African support for these sanctions,” he said. “Obviously, the start has to be with an arms embargo on Zimbabwe, and perhaps even, if political will can be found, submitting the case of Zimbabwe to the international justice system so the leaders of Zimbabwe can be held to account for the abuses in that country.”

While Mr. Mbeki has refused to end his quiet diplomacy with Mr. Mugabe and to denounce his tactics since the March election, other African states have begun to recognize the legitimacy of Mr. Tsvangirai’s complaint. The Zambian president, Levy Mwanawasa, said last week that it was “scandalous” that the Southern African Development Community, a regional group with influence in Zimbabwe, chose to remain silent on the electoral violence there. Also speaking out against Mr. Mugabe is his onetime ally, José Eduardo dos Santos of Angola.

In Washington, Mr. Tupy said he believed the best move for America is to press for a resolution at the U.N. Security Council and quietly prod Zimbabwe’s neighbors to stiffen their resolve. “The United States can best help Zimbabwe by pressuring southern African leaders to take action against Mugabe and pursuing the matter in the U.N. Security Council,” he said.