In effort to boost president Robert Mugabe's campaign, a bus operator attaches an election poster to a passenger bus, 26 Jun 2008

Zimbabwean officials say Friday's presidential runoff will take place as scheduled, despite the withdrawal of opposition leader Morgan Tsvangirai and international calls for the vote to be postponed.

State media Thursday say the Zimbabwe Electoral Commission has decided Mr. Tsvangirai's withdrawal letter, submitted Tuesday, came too late to have legal effect.

In interviews today, Mr. Tsvangirai repeated an offer to negotiate an end to Zimbabwe's political crisis. But he said no talks can take place if the runoff takes place and President Robert Mugabe declares victory.

A chorus of opposition leaders, Western countries and human rights groups say Mugabe supporters have killed dozens and injured hundreds of opposition activists ahead of the run-off.

Mr. Tsvangirai pulled out of the runoff on Sunday, saying the violence made the election impossible. He has stayed at the the Dutch embassy in Harare for most of this week because of safety concerns.

Mr. Tsvangirai's second-in-command, Tendai Biti, was released on bail Thursday after being held for two weeks on charges of treason.

Mr. Tsvangirai had demanded Biti's release as a condition for holding negotiations with the government.

International leaders continue to put pressure on the Mugabe government. U.S. Secretary of State Condoleezza Rice told reporters in Kyoto, Japan today that no run-off could be considered legitimate unless the opposition is involved.

During an appearance in London Wednesday, former South African President Nelson Mandela said there is a "tragic failure of leadership" in Zimbabwe. U.S. President George Bush said the vote appears to be a sham.

Leaders with the regional Southern African Development Community called for the run-off vote to be postponed after an emergency meeting Wednesday in Swaziland's capital, Mbabane.

Meanwhile, Mr. Tsvangirai released a statement late Wednesday denying he had written an article for Britain's Guardian newspaper calling for United Nations peacekeepers in his country.

Some information for this report was provided by AFP, AP and Reuters.