Worries about Zimbabwe's political climate two weeks ahead of the June 27 presidential election run-off continued Friday amid uncertainty of the whereabouts of an opposition politician and a call by African dignitaries for an end to violence.

A group of former African presidents, two former heads of the United Nations, African Nobel laureates and some of the continent's top artists and business leaders have called for an end to violence and intimidation ahead of the election run-off.

In an open letter published Friday, they also called on the Zimbabwean government to restore full access to the country for humanitarian and aid agencies helping the country's people.

'We are deeply troubled by the current reports of intimidation, harassment and violence,' the leaders said.

'It is vital that the appropriate conditions are created so that the presidential run-off is conducted in a peaceful, free and fair manner.

'Only then can the political parties conduct their election campaigning in a way that enables the citizens to express freely their political will.'

The signatories to the letter include former UN secretaries- general Kofi Annan and Boutros Boutros-Ghali and former presidents Jerry Rawlings of Ghana, Abdusalami Abubakar of Nigeria, Joaquim Chissano of Mozambique, Benjamin Mkapa of Tanzania, Ketumile Masire.

Decades-long Zimbabwe President Robert Mugabe is trying to hang on to power in a run-off vote against opposition leader Morgan Tsvangirai.

Tsvangirai was released late Thursday evening after being arrested for a second time in one day, party officials said.

'Morgan is free,' party spokesman George Sibotshiwe said Friday from the central city of Gweru. 'He was released at about midnight. We are all free now.'

But there was uncertainty about MDC secretary general Tendai Biti who was arrested Thursday night on his arrival at Harare International Airport from South Africa.

Bidi had spent two months in South Africa after Zimbabwe's disputed parliamentary and presidential elections. He was handcuffed by about 10 plainclothes police officers as he left the airport just after midday, MDC spokesman Nelson Chamisa said.

Since then, it was not known where Bidi was being held, and his lawyers had not been permitted to see him.

The MDC said Bidi was being accused of high treason - a charge which can bring the death penalty.

According to the MDC, two of Tsvangirai's election campaign vehicles were temporarily seized by police on Friday and searched. The MDC said the two vehicles had already been thoroughly searched five times over the past two days.

Amid the ongoing Harare government actions against the opposition, South African Minister in the Presidency Essop Pahad insisted that free and fair elections in Zimbabwe were still possible despite the deteriorating political situation there.

Speaking to reporters in Cape Town, Pahad cited several countries where the elections were declared free and fair despite outbreaks of violence before the polls.

'The important thing is - if it is held - those who are going to monitor the elections have to make a determination on whether they were free and fair.'

This was presently difficult to determine, he said. It was the role of the South African government to ensure that the violence currently gripping Zimbabwe was stopped.