Nikolai Astreika, the leader of the Partnership group, said police raided the meeting, held at a theater, and arrested 70 members on charges of attending an unauthorized meeting.
Astreika said the group was aware of a prohibition on unauthorized meetings but needed to assemble and hold a founding congress so they could gather the necessary documents to register.
Astreika said police broke up the meeting before it started.
Belarus will hold a presidential election in 2006.
(With agencies)