The United States, Britain, and other countries are denouncing Zimbabwe's government, saying government-sponsored violence forced opposition leader Morgan Tsvangirai to withdraw from next Friday's presidential run-off.

In Washington Sunday, the White House said senseless acts of violence against the opposition by what it called the Mugabe regime must stop. A statement said the U.S. is prepared to go to the United Nations Security Council early in the week to explore additional steps.

U.N. Secretary-General Ban Ki-moon said the circumstances that led to Mr. Tsvangirai's withdrawal do not bode well for the future of democracy in Zimbabwe. The U.N. chief called for the postponement of the election.

British Foreign Secretary David Miliband said state-sponsored violence made the election impossible.  He also said the government cannot be held as a legitimate representative of the Zimbabwean people.

In Brussels, Javier Solana, the European Union's foreign policy chief, said Mr. Tsvangirai's withdrawal was understandable and that the elections have become "a travesty of democracy."

Zambian President Levy Mwanawasa, the current chair of the Southern African Development Community, says Friday's vote should be postponed because Zimbabwe has failed to meet the regional bloc's election standards.

South African President Thabo Mbeki, the SADC's chief mediator between Mr. Mugabe and Mr. Tsvangirai's Movement for Democratic Change, is urging both sides to hold negotiations to resolve the crisis.

Mr. Tsvangirai blamed increasing violence against his supporters as the main reason he pulled out of the run-off.