Labor leaders in Zimbabwe are increasingly being arrested and harassed by police as this month's presidential election approaches, union representatives from the region said Wednesday. Federations of national labor unions representing as many as 15 million people in 11 southern African countries said the abuse of labor activists' rights has multiplied since the first round of presidential elections March 29. Opposition leader Morgan Tsvangirai out-polled President Robert Mugabe in the first round but, according to official results, failed to win the 50 percent plus one vote necessary to avoid a runoff. The second round of voting is scheduled for June 27.

In New York, Zimbabwe's U.N. ambassador, Boniface Chidyauskiku, did not immediately respond to a request for comment about the charge by the union representatives.

Many May Day meetings organized by labor unions in Zimbabwe were forbidden or canceled at the last minute. The country's union leaders were arrested for 12 days and then released on bail, said Alina Rantsolase, of the Congress of South African Trade Unions.

"Police were in their houses, heavily armed, and they had to hand themselves over to the police station," Rantsolase said in Geneva during the annual meeting of the 182 member countries of the U.N.'s International Labor Organization.

Despite their release, the Zimbabwean union leaders were unable to travel to Geneva for the U.N. meeting because of the bail terms.

The opposition, foreign diplomats in Zimbabwe and Zimbabwean and international human rights groups accuse Mugabe of unleashing violence against Tsvangirai's supporters to ensure Mugabe wins the runoff. Zimbabwean government and party spokesmen repeatedly have denied the allegations.

U.N. Secretary-General Ban Ki-moon said Wednesday that he was sending high-ranking envoy Haile Menkerios to Zimbabwe next week to help the nation with the election.

Concerns about the violence in Zimbabwe and the need to deploy neutral international observers were reasons to send a U.N. envoy, Marie Okabe, a U.N. deputy spokeswoman for Ban, has told The Associated Press.

Jan Sithole, general secretary of the Swaziland Federation of Trade Unions, said he was disappointed that the countries in the region and the African Union did not pressure Mugabe to stop his violent campaign.

The Southern African Development Community, or SADC, should immediately put peacekeeping troops in place to restore security and guarantee a peaceful election, Sithole said.

"SADC and AU are not giving this issue the gravity it deserves," said Sithole. The AU is the African Union, an organization of 53 African countries.

The members states of the Southern African Development Community are Angola, Botswana, Congo, Lesotho, Madagascar, Malawi, Mauritius, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe.

Sithole said the regional group should also pressure Mugabe to allow aid workers to resume the distribution of food aid.

He said the government's order last week for humanitarian groups to suspend work, was a "clear strategy" of Mugabe's election campaign. Food was now supplied to Mugabe supporters only, he said.

Meanwhile, the election campaign continues in Zimbabwe.

On Wednesday, Tsvangirai's bus rolled slowly through Harare, the capital, then halted in a cloud of thick black smoke, a symbol of the difficulties he has had campaigning.

Cheering crowds briefly disrupted traffic as the bus moved down Nelson Mandela Avenue. The plan had been to drive 15 miles west to Norton to greet voters and kick off a nationwide tour, but that did not happen.

The smoke appeared to indicate engine trouble.

Before the tour halted, Tsvangirai told reporters on the bus he wanted to liven up his campaign.

"We are heading out to make sure that the message goes home," he said.

His campaign has been beset by violence blamed on Mugabe's forces.

Tsvangirai, who has said he is the target of a military assassination plot, has only been back in Zimbabwe since May 24 after leaving soon after the first round.

Since returning, he has twice been briefly detained by police as he tried to campaign, and police have stopped several attempts to hold rallies. The state-controlled media have all but ignored him in a country where few have access to the Internet or satellite television.