Truckloads of police carrying shields and batons dispersed about 100 demonstrators in the capital, Harare, before the rally could begin.

President Mugabe accused of exceeding authority

Buses carrying would-be marchers, who were also calling for constitutional change, were turned away.

No reason was given for the ban on the demonstration.

Protesters chanted "Say no to intervention". One banner criticising President Mugabe said "Down with Dictators".

Riot police scuffled with marchers who took refuge in Harare's Roman Catholic Cathedral where they sang hymns and knelt in prayer.

The police followed them in to the building, but withdrew after appeals from a priest.

Rally organiser Isaac Maphosa said the government was frightened of democracy and criticism.

"Our constitutional rights for peaceful freedom of movement and expression have been stifled,'' he added.

'An unwinnable and unjustifiable war'

Zimbabwe has sent 6,000 troops to the Democratic Republic of Congo to back President Laurent Kabila's fight against a Tutsi-led rebellion.

Protesters fled to the cathedral

But Morgan Tsvangirai, the head of Zimbabwe's main labour federation, accused President Robert Mugabe of dragging the country into a "stupid, unwinnable and unjustifiable war".

"There is no national consensus on the intervention,'' he added.''This is costing lives and money.''

Zimbabwe has admitted that nine of its soldiers have been killed by Congolese rebels and 16 captured.

No official figure has been given on the cost of the country's operations in the DRC.

But independent estimates say at least US$430,000 would be needed each day to maintain troops and warplanes.

Peace talks have repeatedly failed and Mr Kabila's government has refused to meet with rebel negotiators.

'Kabila's dreadful human rights record'

On Thursday human rights groups in Zimbabwe accused Mr Mugabe of exceeding his constitutional authority by dispatching troops to Congo without asking parliament.

A joint statement said: "The President may declare war on a foreign state carrying out acts prejudicial to the security of Zimbabwe. The situation in [Congo] does not fall into this category.''

President Kabila refuses to talk to rebels

They said many Zimbabweans believed it was wrong to back Mr Kabila who "came to power by military means, has a dreadful human rights record and has singularly failed to allow processes of democratic governance".

The groups said it was widely believed that the mobilisation in Congo was partly to protect the "business opportunities" there of some Zimbabwean politicians and military officers.

The statement demanding the withdrawal of Zimbabwe troops was signed by five groups including the Roman Catholic Commission for Justice and Peace and Zimbabwe Lawyers for Human Rights.

With inflation and unemployment soaring, strikes and civil unrest led to Zimbabwe's first food riots earlier this year in which six people died in police and army gunfire.