Violence erupted last Friday when police fired teargas at opposition leaders and hundreds of demonstrators.
Zimbabwe has banned all demonstrations in the capital Harare for the next two weeks because they are likely to cause public disorder, according to a government notice issued on Thursday.
President Robert Mugabe's opponents have become emboldened by rising public anger over an economic meltdown, cash shortages and high unemployment. Violence erupted last Friday when police fired teargas at opposition leaders and hundreds of demonstrators.
The government notice said the police decided to impose a 14-day ban on all forms of protests in the capital after an assessment by the police commander for Harare, district Chief Superintendent Newbert Saunyama.
People who organise or participate in demonstrations during the period could be fined and jailed for up to a year.
Saunyama declined to comment further, while national police spokeswoman Charity Charamba said she had no details.
Douglas Mwonzora, legal secretary for the National Electoral Reforms Agenda which was planning a march on Friday to present demands to the electoral commission, said the protest had been postponed to September 17.
"The notice is definitely unconstitutional but there is not enough time to challenge it and get a judgment before the demonstration tomorrow, so we decided to postpone it," Mwonzora said.
Opposition parties say the electoral commission is biased in favour of the ruling ZANU-PF and is run by security agencies loyal to Mugabe, charges the commission denies. Mugabe, 92, has led Zimbabwe since independence from Britain in 1980.