At least 100 people, including police, were injured during the clashes in central Dhaka, a police official said.
He said police used tear gas after demonstrators began throwing stones at security forces and smashed several civilian vehicles in the area.
Opposition spokesman Mufazzal Chowdhury Maya said about 200 opposition activists were injured and many were treated at private medical centres.
The clashes broke out after 5,000 protesters tried to overrun barbed-wire barricades that police erected around the Bangladesh Secretariat government buildings.
The protest was sponsored by the main opposition Awami League and its 13 allied parties to demand electoral reform ahead of the country's next polls scheduled for 2007.
They have demanded the resignation of three of the five electoral commissioners for allegedly trying to create new voter lists that would favour the ruling coalition.