Members of the ruling party Zanu PF militia beat unidentified people near the venue of a Movement for Democratic Change party rally in Harare, on Sunday. (Associated Press)

Zimbabwe's opposition leader has pulled out of Friday's run-off election against President Robert Mugabe because of mounting violence and intimidation against opposition candidates.

Morgan Tsvangirai, leader of the Movement for Democratic Change, announced his decision Sunday during a news conference in Zimbabwe's capital, Harare.

He also asked the United Nations "to intervene to restore the rule of law, peace, and the conditions of a free and fair election."

Tsvangirai has accused backers of the ruling Zanu PF party of launching a series of politically motivated attacks since the first round of voting in March.

Independent human rights groups say 85 opposition supporters have died and 1,300 have been beaten or tortured.

Two opposition supporters were reported to have been seriously hurt Sunday after they were beaten while trying to reach a MDC campaign. The opposition said police in full riot gear and soldiers took over the site of the rally.

'Credible election is impossible'

"Conditions as of today do not permit the holding of a credible poll," 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."

Tsvangirai said he would put forward new proposals by Wednesday on how to 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.

During the March 29 presidential election, Tsvangirai won 47 per cent of the votes and Mugabe 43 per cent, neither winning an absolute majority.

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."

At a rally in the western city of Bulawayo on Friday, Mugabe — who has held power since the country gained independence from Britain in 1980 — said that the opposition was lying about the violence and said everywhere he visited was peaceful.

Mugabe, 84, was lauded early in his rule for campaigning for racial reconciliation. But in recent years, his administration has seen inflation skyrocket and agricultural production plummet.

The economic slide began in 2000 when the government began to seize thousands of white-owned commercial farms to reallocate the property to landless blacks, many of them ruling party loyalists.

With files from the Associated Press