Morgan Tsvangirai,the MDC leader, is seeking to topple Robert Mugabe's 28-year reign over the country in the June 27 vote.
Rally ban rationale
According to a letter signed by a superintendent of police, a copy of which was read to the AFP news agency, authorities based their decision to bar the rallies on MDC statements concerning assassination threats.
"The MDC has communicated far and wide, very loudly for that matter, that the lives of some of your politicians are under severe threat from targeted assassination," the letter said.
"We as police are still battling to assess the assassination threats levels, trying to isolate and eliminate such threats.
"Our continued investigations so far have failed to confirm your party's allegations but still we are not prepared to take any chances by exposing you to the public who may be possible assassins."
Having spent some six weeks out of the country after the first round of voting on March 29, Tsvangirai delayed his return until a fortnight ago after the MDC said it had uncovered a plot to assassinate him.
He defeated Mugabe in the first round of the election, but with an official vote total just short of an outright majority.
The MDC has so far been able to stage only a handful of rallies and Tsvangirai was detained twice by police this week.
George Sibotshiwe, a spokesman for Tsvangirai, told Al Jazeera on Friday that Tsvangirai's arrest was an attempt to derail his election campaign.
"This is a deliberate strategy by Zanu-PF to disrupt Mr Tsvangirai's campaign and prevent him from having access to the Zimbabwean people," he said.
Ban on aid groups