Zimbabwean presidential challenger Morgan Tsvangirai plans to announce Saturday whether he will take part in a run-off election against the incumbent, Robert Mugabe.

Officials from the opposition Movement for Democratic Change party say Mr. Tsvangirai will make a "definitive statement" on his intentions at a news conference in South Africa's capital, Pretoria.

The candidate has said in the past he will not take part in the run-off unless it is observed by international monitors.

Mr. Tsvangirai says he won an outright victory in Zimbabwe's March 29 presidential election. The electoral commission says while he won the most votes, he fell short of a majority.

Friday, U.S. Secretary of State Condoleezza Rice telephoned the presidents of several of Zimbabwe's neighbors to try to get them to use their influence to help resolve the situation.

State Department spokesman Sean McCormack said Rice called the presidents of Botswana, Zambia, and Tanzania, as well as former U.N. Secretary-General Kofi Annan.

South African President Thabo Mbeki traveled to Zimbabwe Friday and met with President Robert Mugabe. He did not meet with the opposition. The opposition has criticized Mr. Mbeki for refusing to take a tough line on President Mugabe.

The opposition and human rights groups accuse Mugabe loyalists are trying to intimidate MDC supporters ahead of a possible runoff election.

Friday, a Zimbabwean doctors' group said its members have attended to more than 900 cases of torture and assault since the election. The Zimbabwe Association of Doctors for Human Rights says that figure is likely a fraction of the real number of victims, as many incidents go unreported. ZANU-PF has rejected allegations that it is responsible for the violence.

Also Friday, the country's main labor federation, the Zimbabwe Congress of Trade Unions, say its two top leaders have been arrested.

If Mr. Tsvangirai does not contest the run-off, Mr. Mugabe will remain president by default, extending his 28-year rule over Zimbabwe. 

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