The Zimbabwe prime minister's party says police took three of its senior officials from their homes early on Sunday but gave no reasons for their action.

A prominent human rights lawyer, Beatrice Mtetwa, was a fourth person taken to a suburban office of Prime Minister Morgan Tsvangirai's media and communications department for questioning.

The party said in a statement the three party officials, including senior attorney Thabani Moyo, work on the prime minister's staff.

Voting in a referendum on a new constitution ended late Saturday. All main party leaders called for a "Yes" vote on constitutional reforms. Another official of the former opposition Movement for Democratic Change was arrested before polling began Saturday.

Police enforced a clampdown on rights and pro-democracy groups in the run-up to the referendum vote.