The decision came amid confusion over whether President Robert Mugabe would attend a regional summit on the crisis, in Zambia at the weekend.
The results of the election, held 13 days ago, have yet to be released.
The opposition Movement for Democratic Change has called for a strike starting on Tuesday to pressure the authorities.
Mugabe has deployed the military in the provinces, in the districts... In other words, he is creating a new electoral environment that is neither free nor fair
Movement for Democratic Change
The MDC's leader, Morgan Tsvangirai has said he will not take part in any second, run-off presidential poll with Mr Mugabe.......
The ban on political rallies across the country comes two days before a planned demonstration by the MDC in the capital Harare.
No official reason has yet been given by the government for the ban, but state radio suggested that the police did not have sufficient manpower.