Updated

An alliance of rights organizations says police in Zimbabwe have raided the offices of an independent election support group searching for alleged subversive materials.

The Crisis Coalition said Tuesday 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.

The coalition said no further information was immediately available.

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.