"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," Carlton Carroll, a White House assistant press secretary, said in a statement.
U.N. Secretary-General Ban Ki-Moon called the situation in Zimbabwe "deeply distressing."
"The campaign of violence and intimidation that has marred this election has done a great disservice to the people of the country and must end immediately," he said in a statement.
The United Nations is prepared to work urgently with Southern African Development Community and the African Union to help resolve the impasse, Ban said.
Thabo Mbeki, president of leading regional power South Africa, said he would encourage Mugabe and Tsvangirai to discuss the political crisis.
"From our point of view it is still necessary that the political leadership of Zimbabwe should get together and find a solution to the challenges that face Zimbabwe," said Mbeki who is mandated by regional bloc SADC to mediate between the opposition and the ZANU-PF.
Zambian President Levy Mwanawasa, the current SADC chairman, said the run-off should be postponed "to avert a catastrophe in this region," a view echoed by Ban.
Tsvangirai, who himself had been detained by police five times while campaigning, said 86 MDC supporters had been killed and 200,000 displaced from their homes.
Mugabe has vowed never to turn over power to the opposition, which he brands a puppet of Britain and the United States. He has in the past denied that his security forces have been responsible for brutal actions.
Once Tsvangirai pull outs, Mugabe would then be sworn in for another five-year term. But he could face difficulties governing because the MDC won control of the parliament in a March election.
Former colonial power Britain said the people had deserted Mugabe.
"We have reached an absolutely critical moment in the drive by the people of Zimbabwe to rid themselves of the tyrannical rule of Robert Mugabe," Foreign Secretary David Miliband said.
The MDC earlier said that thousands of youth militia loyal to Mugabe poured into an MDC rally in Harare. Armed with iron bars and sticks, they beat journalists and forced election observers to flee, the MDC said.
But Zimbabwe's government denied this.
"We do not accept that those people were ZANU-PF. We know the MDC has been giving its thugs ZANU-PF regalia to create the impression that we are behind the violence," Justice Minister Patrick Chinamasa said.
