A U.N. official in Zimbabwe says post-election violence is increasing, and he largely blames President Robert Mugabe's supporters for the attacks. Peta Thornycroft reports that political tensions are rising as opposition leader Morgan Tsvangirai prepares to return to Zimbabwe for a runoff election campaign.

The United Nations is warning that post-election violence is reaching crisis levels in Zimbabwe.

In a rare public statement, U.N. resident representative Agustino Zacarias says the violence is largely inflicted on rural supporters of the Movement for Democratic Change, which beat the ruling ZANU-PF in the March parliamentary election. He said non-governmental organizations and civil rights defenders were also being targeted

Zacarias blames most of the violence on groups loyal to ZANU-PF. But he said there were some incidents in which MDC people had perpetrated acts of violence.

He said this unrest is preventing U.N. humanitarian agencies from reaching people in need and had forced it to reduce operations.

Shortly after the U.N. official made his statement, police stopped a convoy of ambassadors on a tour to a hospital in Mvurwi about 80 kilometers north of Harare.

According to journalists traveling with the diplomats that included U.S. Ambassador to Zimbabwe James McGee, the police demanded that the group provide proof they had permission to visit the hospital. The group was visiting victims of political violence who were being treated at a hospital in Mvurwi, and insisted they be let through.

Robert Mugabe makes speech during independence celebrations in Harare, 18 Apr 2008

In an open letter published Monday in the state-controlled daily Herald newspaper, McGee accused President Robert Mugabe's ZANU-PF of orchestrating violence to intimidate MDC supporters before a runoff 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 hospitalized since the first round of voting March 29.

The Herald accused the U.S. ambassador of "very scandalous acts" and of breaching diplomatic protocol by speaking out on the violence.

Zimbabwean Movement for Democratic change (MDC) president Morgan Tsvangirai speaks on 10 May 2008

Opposition leader Morgan Tsvangirai won the most votes in the first round, but not the simple majority needed to avoid a runoff, according to official results.

Hundreds of MDC supporters, activists and party workers as well as human rights lawyers, journalists, students, trade unionists and non-governmental organization workers have been arrested since the elections.

The Southern African Development Community's Zimbabwe mediator, South Africa President Thabo Mbeki, has said nothing about the violence since he had lengthy meetings with President Mugabe last Friday.

Morgan Tsvangirai said Saturday he would be returning to Zimbabwe this week for a victory tour and to campaign for the presidential runoff, which was supposed to take place by May 24 but has been postponed to an unknown date.