The Zimbabwean opposition says it will defy a police ban and hold a protest march in the capital on Wednesday.
People are facing economic meltdown in Zimbabwe
The Movement for Democratic Change said the protest was aimed at highlighting the economic crisis in the country.
It also wants a new constitution before parliamentary and presidential elections due in March.
The MDC warned earlier this month that it would boycott the polls if it was not satisfied with preparations to ensure they would be free and fair.
"We're marching because our people are suffering... there's no water, no electricity," Tendai Biti, secretary-general of the MDC faction that is led by Morgan Tsvangirai, told reporters.
"The government is totally bereft and bankrupt of any capacity to govern this country."
Police spokesman Wayne Bvudzijena said the police had reversed its decision to allow the march saying they feared it could degenerate into rioting.
"The march is not going to be peaceful as the MDC told us, so we have cancelled it," he told AFP news agency.
"As the police, we cannot stand idly while people break the law. Anyone who participates in this march will be arrested."
But Mr Biti rejected the ban.
"The illegality of the action by the police is in blatant disregard of the new provisions of the Public Order and Security Act."
The government last year said it would relax tough security laws which have been used to block political rallies.
The MDC has said it called the march to test the government's commitment to political reform.
South African President Thabo Mbeki is trying to mediate between the government and the opposition to try and resolve differences.