A number of people were injured when thousands protesting against the occupation of white-owned farms were attacked.

This is a desperate act ... they (the government) want to force people to support them

MDC leader, Morgan Tsvangirai

Correspondents say the attackers - many of them veterans from the war of independence - singled out white people.

Earlier, police mounted roadblocks on major routes into Harare in an attempt to stop the protest, which had been called by the National Constitutional Assembly (NCA) in support of the rule of law, and in opposition to the recent wave of invasions of white-owned farms.

Demonstrators who did manage to reach the starting point were dispersed. But they regrouped, gathering several thousand followers - including elderly couples and families walking hand-in-hand - and marched peacefully through the capital.

The violence erupted when the marchers were confronted by about 200 government supporters armed with sticks and stones.

The BBC's correspondent in Harare, Grant Ferrett, says police stood back as the anti-government marchers were attacked, intervening with tear gas only after a number of people were injured.

He says that for the next 30 minutes, government supporters swaggered through the streets beating people, apparently at random.

Protester Jenny Yon said: "I stood with a poster which said 'no violence', and they beat me."

Further demonstration planned

By early afternoon, the city centre was quiet again, but tension remains high with plans for a further demonstration later in the day.

Several thousand joined anti-government protest

Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC), a broad-based party that dominates the NCA, said: "This is a desperate act ... they (the government) want to force people to support them.

"Obviously they have found out from the referendum the writing was really clear about where the people stand.

"We are still planning for elections this year, but it may be too unrealistic given the amount of violence that is being agitated from the state."

Zimbabwe's High Commissioner to the UK, Simbarashe Mumbengegwi, denied police looked on as the attacks took place.

"You had a situation where you had two competing interests. There is no suggestion the police were standing in front of those who are anti-government.

"Whenever you have such an explosive situation developing, the police have a responsibility to move in, and they did move in," he said.

Government crisis

Mr Mugabe's government is embroiled in its worst economic and political crisis since independence in 1980, and in February it lost a referendum on constitutional reform.

The constitution that was rejected would have given Mr Mugabe the right to seize white farms for redistribution to blacks without the need for compensation.

More than 600 white-owned farms have been seized in support of Mugabe's plans.

Elections, originally set for April, have been postponed until May at the earliest. Political analysts say the ruling party, Zanu-PF faces an unprecedented challenge from the new opposition MDC.

Britain has said it will take in 20,000 white Zimbabweans if they are subjected to further violence.