AFP, HARARE - Police on Friday raided the offices of Zimbabwe's main independent election observation group in the capital Harare, a source at the Zimbabwe Election Support Network said.

"Police are searching our offices, they are flipping through files and documents but they have not taken anything," said the source, who requested anonymity.

"The police officers arrived at around 11:00am (0900 GMT) and are going through documents in all the offices while being monitored by the programmes officer," the source said.

News of the raid came shortly after riot police rounded up dozens of supporters of the main opposition party, the Movement for Democratic Change (MDC), in a raid on its Harare headquarters.

The Zimbabwe Election Support Network, a group of non-governmental organisations, is considered the only reliable independent source of information about the March 29 elections.

The group has named MDC leader Morgan Tsvangirai as the winner over veteran Zimbabwean President Robert Mugabe in the first round of the presidential election, from which no official results have yet been released.

The top US envoy for Africa, Assistant Secretary of State Jendayi Frazer, who on Thursday declared Tsvangirai the clear winner of the election, based her conclusions on results given by the networ.