Zimbabwean police raided the headquarters of the opposition and independent election observers on Friday as the United States ratcheted up the diplomatic pressure on veteran President Robert Mugabe.

Dozens of supporters of the Movement for Democratic Change were held in the raid on its offices in downtown Harare in an operation that police said was designed to find the perpetrators of recent arson attacks.

Sources at the Zimbabwe Election Support Network (ZESN) said police had confiscated a series of documents after rifling through their offices. There were no arrests.

The raids added to mounting tensions in the troubled southern African nation where the final outcome of joint presidential and parliamentary elections held on March 29 is still unknown.

"This is systematic harrassment," said chief opposition spokesperson Nelson Chamisa after the riot squad rounded up a busload of MDC supporters and drove them away for questioning at central police headquarters.

"What is clear is that these people are desperate and they can do anything."

National police spokesman Wayne Bvudzijena said officers had been searching for the perpetrators of a number of recent violent incidents.

"We do know that too many people have taken shelter at the MDC offices and we suspect some of them have committed arson attacks in rural areas and have come to hide in some safe houses," he told AFP.

There was no immediate word from the police about the raid on the ZESN, but a source in the organisation said that various, including old presentations and speeches, had been seized during a four-hour search of their premises.

Figures from the ZESN were cited by the main US envoy for Africa, Assistant Secretary of State Jendayi Frazer, when she declared on a visit to South Africa Thursday that MDC leader Morgan Tsvangirai had clearly beaten Mugabe.

After meeting with South African government officials, Frazer then met with Tsvangirai to discuss his recent talks with regional leaders and growing reports of violence on the ground.

"We assured the MDC that we would look at additional international action to address, and bring attention to, the evolving human rights and humanitarian crisis in Zimbabwe," Frazer said, according to a statement from the US embassy.

Frazer was also due to meet in Luanda on Friday with Angolan President Jose Eduardo dos Santos, a long-time ally of Mugabe.

The authorities in Zimbabwe have consistently maintained that the delay in announcing the presidential election result is down to a meticulous process of collating and verifying ballot papers and have hit out at outside interference.

In a speech in the southern city of Bulawayo on Friday, Mugabe avoided any direct reference to the election but stressed the former British colony would not be ordered around.

"Zimbabwe has a history and heritage and it will never be afraid. Zimbabwe is not for sale and Zimbabwe will never be a colony again," he said at the opening of a trade fair.

Mugabe, who has ruled uninterrupted since independence in 1980, has presided over a dramatic collapse of a country which has the world's higest inflation rate, officially put at 165,000 percent.

Once considered the bread basket of the region, even bread and sugar are now in short supply while average life expectancy has slumped to 36 years of age.

AFP