The main opposition party in Zimbabwe has warned President Robert Mugabe that it will organise more protests, after police cracked down on opposition militants in two cities at the weekend.

The leader of the Movement for Democratic Change, Morgan Tsvangirai, said that the government should brace itself for more demonstrations.

Mass action is never impossible, this is only going to strengthen our resolve

MDC's Tendai Biti

He did not specify when they would take place, but said that it was a matter of time before action was taken.

A rally by MDC supporters was dispersed by police in the capital, Harare, on Sunday.

Police say at least 80 MDC youths and officials have been arrested and will be charged for violating the law on security and order.

They are expected to appear in court on Tuesday.

Last week, state-run media reported that President Mugabe had put security forces on high alert to crush any mass demonstrations calling for a re-run of the March presidential elections.

Clash

Hundreds of ruling party militants clashed with MDC supporters in Zimbabwe's second city, Bulawayo, early on Monday.

Mugabe signed the media law in March

Zanu-PF militias deployed in the townships last week, and started harassing MDC supporters overnight, raiding their houses and beating them up.

No casualties have been reported in the clashes, in which hundreds of people were involved on both sides, according to residents.

Police and soldiers were deployed in the townships, but there were still a few pockets of resistance a few hours after the clashes started.

'Struggle for democracy'

The arrests in Harare took place at a demonstration held to commemorate the 1976 Soweto uprising.

Riot police used tear gas and clubs, and fired shots in the air to disperse hundreds of opposition supporters.

An MDC member, Tendai Biti, vowed to keep up the struggle for democracy in Zimbabwe.

"Mass action is never impossible, this is only going to strengthen our resolve," he told AP news agency.

Media restrictions Sunday's opposition rally also is reported to have led to the arrest of three journalists.

Geoff Nyarota, the editor of the independent Daily News, told the BBC that three of his journalists had been beaten up by the police as they tried to cover the march in Harare.

Meldrum's trial is expected to resume on Monday

Mr Nyarota said the reporters were then taken to Harare central police station. One of them is in severe pain from injuries to his arm.

At the weekend, the Zimbabwean government introduced more restrictions to the work of national and international reporters in what critics see as an attempt to limit foreign media in the country.

An amendment to the new media law says that foreign media companies will need to pay the equivalent of a total of $12,000 US to be registered.

American journalist Andrew Meldrum, who works for the British newspaper, The Guardian, is already facing trial for publishing falsehoods.

He could face a hefty fine or a prison sentence of up to two years.