An alliance of rights organizations said Zimbabwean police on Tuesday raided the offices of an independent election support group searching for alleged subversive materials.
The Crisis Coalition said police knocked down a gate to get into the suburban Harare offices of the Zimbabwe Election Support Network and escorted visitors out of the building before starting their search.
No further information was immediately available, the coalition said.
Zimbabwe is set to vote in a referendum on a new constitution March 16, followed by national elections later in the year.
In a crackdown in the past week, three other pro-democracy groups have been raided by police, one also accused of subversion and possessing unauthorized recording equipment and voter education display materials.
Zimbabwe's President Robert Mugabe announced the referendum date on Friday.
The election support group said in a statement Monday the fixing of such an early date for the referendum denied citizens "a meaningful opportunity" to learn about the 160-page draft constitution to make an informed choice to vote 'Yes' or 'No' in the referendum.
It said the official Zimbabwe Electoral Commission did not have enough time to organize a credible vote that would be recognized by regional governments and follow international voting standards and guidelines.
Later Tuesday, the National Constitutional Assembly, a group opposed to rapid adoption of the new constitution, filed a High Court suit to ask for an order deferring the vote for at least another two months.
Andrew Makoni, the group's attorney, said distribution of the draft document only began Monday and a translation from English into local languages, Shona and Ndebele, for the nation's 6 million voters has not started.
"People will not have ample time to study it," he said.
He said the court has yet to set a hearing day for the case.
Rights groups have criticized police for harassment of activists that has included a raid on the respected Zimbabwe Peace Project last week, the tear gassing and baton charge of women marchers on Valentine's Day and the arrests Saturday of a pastor and community campaigners after a meeting in the town of Chegutu to encourage unregistered adults to add their names to voters lists.
The Crisis Coalition said Tuesday the latest police action "appears to be a calculated state sponsored move to inculcate fear in groups doing election related work" as polling approaches.
Veritas, a legal research and advocacy group, said the referendum is needed to pave the way for elections to end a shaky coalition between Mugabe and the former opposition of Prime Minister Morgan Tsvangirai brokered by regional leaders after violent and disputed polls in 2008.
"But the nation's constitution is too important to cut the time necessary for proper consideration just to speed up elections," it said.
There were also legal and technical issues as well as funding still to be raised that made a March 16 "totally unreasonable."
The draft was completed Feb.6 after three years canvassing for the opinions of electors amid bitter bickering and disputes between political leaders.
Many already believe the main political parties, who have called for a 'Yes' vote, hijacked final drafting of the new constitution.
Veritas said a hasty referendum creates the impression that the draft serves the interests of political parties not the views of the nation.
"This would negate a feeling of ownership by the people," Veritas said.