Johannesburg/Harare  - President Robert Mugabe threatened to arrest Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC) and his opponent in the run-off presidential elections June 27, just as a special United Nations envoy arrived in Harare.

A UN spokesman in Zimbabwe confirmed that Haile Menkerios, the world body's assistant secretary-general for political affairs, arrived in Harare late Monday, following Mugabe's agreement to a request from UN secretary-general Bank-Ki Moon to send an envoy to investigate the crisis in Zimbabwe.

No details of his plans were immediately available from the UN office in Harare.

Meanwhile, in the state-controlled Herald newspaper Tuesday, Mugabe claimed that MDC supporters were carrying out "arson, kidnappings and violence on people, especially ZANU(PF) (Mugabe's party)."

There has been no independent confirmation of claims by Mugabe and the state press of violence perpetrated by members of the pro- democracy party.

Instead, churches, human rights groups and doctors have confirmed a wave of killings, brutal assaults, torture, abduction and driving people out of their homes in which the victims assert their assailants have been ZANU(PF), except in a small minority of cases.

Mugabe said there was "an organized system of violence aimed at disturbing law and order."

He said the government would "soon invoke what is known as (the law of) vicarious responsibility and liability which means that we will hold them responsible for the violence across the country."

The law was usually used against the state, when state agents were committing crimes, and top government officers were held responsible for the actions of their juniors, said Irene Petras of Zimbabwe Lawyers for Human Rights.

Already, the MDC secretary-general, Tendai Biti, is under arrest on charges of treason. Tsvangirai has been arrested without charge at least six times in the last two weeks as he attempted to campaign around the country, but was detained for no longer than 12 hours.

About six MDC MPs have been arrested since the first-round elections on March 29.

One of them, advocate and former administrative court judge Eric Matinenga, is in police cells in his constituency in south-east Zimbabwe after being arrested twice on "inciting violence" charges, which were dismissed by a magistrate.

His lawyers are in the process of securing a court order to have police commissioner Augustine Chihuri arrested for contempt of court for disobeying high court orders over a week ago for Matinenga's "immediate release."

Observers say Mugabe's rhetoric in campaign speeches is growing increasingly violent, with him declaring yesterday that he would not allow "a mere X" in the June 27 election to make him give up power.

Aged 84, Mugabe has been in power since independence in 1980 and the country's economy is on the brink of grinding to a halt, according to economists.

In Tuesday's Herald, he told his audience that "there is hunger in this country and there are no commodities but you cannot sell the country for that." He asserts that Tsvangirai "cannot be allowed to win because he will hand the country to the British and the Americans.

"You decide for yourselves, to vote for war, or for people who work for the development of the country," he said Tuesday.

The Nelson Mandela Foundation signed an open letter calling for free and fair elections in Zimbabwe, the foundation said on Tuesday.

The letter was also endorsed by other African leaders, organizations and individuals in Africa.

"We are profoundly concerned by the situation in Zimbabwe and would like to join all freedom-loving people who have added their voices to the growing call for true democracy, " said CEO of the Foundation Achmat Dangor.

The letter stated that it was crucial for the interests of both Zimbabwe and Africa that the upcoming elections were free and fair.

"As Africans we consider the forthcoming elections to be critical. We are aware of the attention of the world. More significantly we are conscious of the huge number of Africans who want to see a stable, democratic and peaceful Zimbabwe," the letter 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,"the letter added. (dpa)