Zimbabwe's main opposition party says four of its activists have been killed by supporters of President Robert Mugabe, as violence escalates ahead of the June 27th presidential run-off election.

Movement for Democratic Change spokesman Nelson Chamisa says the bodies of the four activists were found near Harare Thursday, two days after they were abducted.

The deaths were reported a day after South African President Thabo Mbeki met separately with President Mugabe and opposition leader Morgan Tsvangirai, trying to mediate the political crisis.

Two South African newspapers, "Business Day" and "The Star," report that Mr. Mbeki asked Mr. Mugabe to cancel the run-off, and for the two rivals to form a national unity government. 

But in an interview today with VOA's Studio 7 Zimbabwe, Mr. Tsvangirai dismissed those reports, saying the South African president told him he has no plan to end the crisis in Zimbabwe.

The MDC accuses Mugabe supporters of waging a campaign of violence and intimidation to increase the president's chance of victory in the run-off.  

The MDC says about 70 of its supporters have been killed.  The president and ruling ZANU-PF party blame the MDC for the violence.

Police have detained Mr. Tsvangirai at least five times this month as he tried to campaign for the run-off.  Authorities have also arrested the MDC's number-two official, Tendai Biti.

Biti, a strong critic of the president, appeared in a Harare courtroom Thursday as police filed papers accusing him of treason.  If convicted, he could face the death penalty.

Mr. Tsvangirai defeated Mr. Mugabe in elections March 29, but official results showed him falling short of the majority needed to avoid a run-off.

Mr. Mugabe has strongly suggested that a Tsvangirai victory would lead to civil war in Zimbabwe.  The 84-year-old president has ruled the country since it won independence from Britain in 1980.

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