Police on Tuesday forcibly entered the offices of the Zimbabwe Election Support Network by breaking part of the perimeter wall and confiscated documents, a human rights lawyer said on Wednesday.
No arrests were made.
"They had a search warrant to search for subversive material, documents, gadgets, recordings and to look for illegal immigrants," Zimbabwe Lawyers for Human Rights spokesperson Kumbirai Mafunda told Agence France-Presse.
Police have in recent weeks targeted non-governmental organisations as the country readies for a crucial constitutional referendum on March 16.
Meantime a pro-democracy group opposed to the draft constitution on Tuesday filed for an urgent court order to defer the referendum to allow for more time to study the text.
The National Constitutional Assembly argued that the general public were not involved in the drafting of the law, while their contributions were not captured in the text.
Scrutinise the constitution
"It is unfair that they should be given just about a month to make up their minds," the assembly said in the court documents.
"It is therefore necessary that the people be given an adequate opportunity to scrutinise the constitution in order to make informed choices."
A new constitution is a key reform for new elections to choose a successor to an uneasy powersharing government formed nearly four years ago by long-ruling President Robert Mugabe and his nemesis Morgan Tsvangirai, now prime minister.
Elections are due in July.
Meanwhile Zimbabwe Election Support Network, a household name in Zimbabwe's election monitoring, was also targeted in past elections. The network exposed the shambolic state of electoral rolls, which it also claimed were stuffed with the names of dead people and children below voting age.
A week ago, detectives ransacked the offices of a civic group called the Zimbabwe Peace Project and seized documents and CDs.
Detained activists
At the weekend police also detained three rights activists in the farming town of Chegutu, west of the capital, on charges of holding an unsanctioned meeting to discuss the constitution.
The new charter is designed to pave the way for new elections in July, which are set to end the power-sharing government between Tsvangirai and Mugabe.
With the referendum just weeks away, political tensions are building up and activists increasingly targeted.
Activist group the Crisis in Zimbabwe Coalition said the raids appear "to be a calculated state sponsored move to inculcate fear in civil society organisations' doing election related" work.
Authorities have in the past threatened to revoke licences for groups deemed to be opposed to Mugabe's policies.
Tsvangirai's Movement for Democratic Change party has urged the police to stop harassing rights activists and party supporters. – AFP