HARARE, Zimbabwe - The United Nations says post-election violence in Zimbabwe is nearing crisis levels.

UN spokesman Agustino Zacarias says "several people" have been killed, hundreds hospitalized and more displaced.

He blames the violence on "elements of the security forces, youth militias and war veterans and gangs of supporters" of President Robert Mugabe.

Zacarias says there is an emerging pattern of violence against rural supporters of the opposition Movement for Democratic Change.

Non-governmental organizations and civil rights defenders are also being targeted.

He says that is preventing UN humanitarian agencies from reaching people in need and forcing them to scale down operations.

Meanwhile, police on Tuesday stopped a convoy of ambassadors on a tour to investigate attacks on opposition supporters.

Police demanded that the diplomats - including the British and U.S. envoys, whose countries have had tense relations with the Harare government - prove they had official permission to visit hospitals and an alleged torture camp.

U.S. Ambassador James McGee insisted the convoy be allowed through. It was allowed to pass after about an hour, led by a police car, but it was unclear where it was headed.

Japanese, EU, Dutch and Tanzanian envoys and an Associated Press reporter also were in the convoy, which was stopped on the edge of Harare as it was returning to the capital.

The incident happened just after Zacarias, the UN resident representative in Zimbabwe, spoke of the escalating tensions in both rural and urban areas.

"There are indications that the level of violence is escalating in all these areas and could reach crisis levels," he said.

The UN representative's comments echoed those made by McGee in an open letter published Monday in the state-run Herald newspaper.

The letter accused Mugabe's party of orchestrating violence to intimidate opposition supporters before the electoral commission holds a second round to the March 29 presidential election.

McGee said the U.S. government has confirmed at least 20 deaths and more than 700 incidents of violence resulting in more than 200 people being hospitalized since the first vote.

The paper in turn blasted McGee, accusing him of "very scandalous acts" and of breaching diplomatic procedure.

Opposition leader Morgan Tsvangirai won the most votes in the first round, but not by enough to avoid a run-off, according to official results.

Observers have questioned whether a second round would be free and fair with the opposition unable to campaign freely because of attacks and threats. No date for a run-off has been set.