HARARE, Zimbabwe — Opposition leader Morgan Tsvangirai said today he is pulling out of this week’s presidential runoff because of mounting violence and intimidation against his supporters.

Mr. Tsvangirai announced his decision during a news conference at Zimbabwe’s capital after thousands of ruling party militants blockaded the site of the opposition’s main campaign rally.

“Conditions as of today do not permit the holding of a credible poll,” Mr. Tsvangirai said. “Given the totality of these circumstances, we believe a credible election is impossible. We can’t ask the people to cast their vote on June 27 when that vote will cost their lives. We will no longer participate in this violent sham of an election.”

Mr. Tsvangirai said he would put forward new proposals by Wednesday on how take the country forward. He did not provide any details about what the proposals would include.

“Our victory is certain, but it can only be delayed,” he said.

Mr. Tsvangirai had hoped to address his main campaign rally ahead of the runoff against 84-year-old President Robert Mugabe, who has held power since independence from Britain in 1980.

But the Movement for Democratic Change claimed the militants were beating opposition supporters who were trying to reach the venue Sunday and said at least two were seriously injured.

It said the militants attacked journalists and forced African election monitors near the rally site to flee. Election monitors could not immediately be reached for comment and there was no independent confirmation of the opposition claims.

Mr. Tsvangirai won the March 29 vote but not by an absolute majority. Campaigning for the first round election was generally peaceful, but the runoff has been overshadowed by violence and intimidation, especially in rural areas.

Independent human rights groups say 85 people have died and tens of thousands have been displaced from their homes, most of them opposition supporters.

Mr. Tsvangirai’s attempts to tour the country have been stymied by police at roadblocks, and the state-controlled media have banned opposition advertisements, claiming they “contain inappropriate language and information.” The media cited one ad that claimed that Mr. Tsvangirai won the election, “which is not the case, hence the runoff.”

The opposition party’s No. 2, Tendai Biti, was arrested within minutes of his return from South Africa last week and is being held on treason charges.

“It is evident that the Mugabe regime has disregarded regional and continental opinion that has been calling for an end to disruption of MDC election campaign programs, state sanctioned brutality, violence and harassment of the people of Zimbabwe,” the opposition said in a statement.

At a rally in the western city of Bulawayo on Friday, Mr. Mugabe said that the opposition was lying about the violence and said everywhere he visited was peaceful. His powerful police chief pinned the blame firmly on the opposition and said that police would clamp down.

Mr. Mugabe was lauded early in his rule for campaigning for racial reconciliation. But in recent years, he has been accused of ruining the economy and holding onto power through fraud and intimidation.

The economic slide of what was once the region’s breadbasket has been blamed on the collapse of the key agriculture sector after often-violent seizures of farmland from whites.

Mr. Mugabe claimed he ordered the seizures, begun in 2002, to benefit poor blacks. But many of the farms instead went to his loyalists.