Riot police, helicopters and armoured personnel carriers have moved on Pearl Square in the Bahraini capital Manama.
Clashes broke out between security forces and protesters as police cleared pro-democracy demonstrators from makeshift camps.
Helicopters swooped overhead and tear gas rained down as youths confronted police. As many as four people are reported dead.
The violence comes a day after Bahrain’s king declared martial law as the government battles to quash an uprising by the island’s Shi’ite majority.
It is unclear if Gulf Arab forces invited in by the monarchy were involved in the operation.
The United States, a close ally of Bahrain and Saudi Arabia has called for calm.
Bahrain hosts the US Navy’s Fifth Fleet.