HARARE, Zimbabwe - A presidential election runoff between Zimbabwe's President Robert Mugabe and opposition leader Morgan Tsvangirai will be held June 27, the electoral commission said in an announcement published Friday.

Tsvangirai claims he won the March 29 presidential race outright, beating Mugabe and two other candidates. But official results released May 2, more than a month after the poll, show he did not win enough votes to avoid a second round against Mugabe.

The electoral commission announced the runoff date in the government gazette, saying: "It is hereby notified that the Zimbabwe electoral commission with the approval of the minister of justice made the following notice: a poll shall be taken on Friday, June 27, 2008, for the purpose of electing a person to the office of president."

The commission had extended the deadline for holding the runoff from the legally required 21 days to 90 days after election results were announced. The opposition, which wanted a May 23 runoff, has said the decision to extend the date was "irresponsible."

George Chiweshe, chairman of the Zimbabwe Electoral Commission, said in an interview Friday with the Herald newspaper, a government mouthpiece, that they needed "substantial" resources to hold the runoff.

"The runoff is a full election and just as big as any general election," he was quoted as saying. "Some of the resources were depleted during the first election, so we need more time to prepare for the runoff."

The opposition as well as local and international human rights groups have accused Mugabe's party of using delays to mount a campaign of violence and intimidation against opposition supporters.

Tendai Biti, secretary-general for Tsvangirai's Movement for Democratic Change, said Thursday that violence was intensifying and now affecting "some of the key pillars of our structure." His party said in a statement that 33 of its supporters and activists had been killed in postelection violence.

Speaking to reporters in South Africa, Biti said the opposition remained determined to participate in the runoff. He also said he and Tsvangirai, who have been out of the country since shortly after the March 29 election, would soon be returning to Zimbabwe.

Biti noted that this weekend, his party planned a campaign rally and a caucus of members elected to parliament, indicating Tsvangirai would be in Zimbabwe for those events.

Tsvangirai's party won control of parliament in legislative elections held alongside the presidential vote. It was the first time since independence that Mugabe's party lost control of parliament.

Speaking in Belfast, North Ireland at an international conference of liberal party members on Friday, Tsvangirai accused Mugabe of clinging to power and said the new parliament can no longer wait to begin governing the country.

"My party, the Movement for Democratic Change, is a government-in-waiting which is not going to wait any longer," Morgan Tsvangirai said at a political conference in Belfast. He said the first act of the new parliament would be to create a new constitution.

"The crisis in our country demands that we begin to implement our legislative agenda," Tsvangirai said. "While Mugabe and his cronies try desperately to cling on to power, we will begin to engage and work with the people."