Helicopters flew overhead and Bahraini riot police fired teargas on Wednesday as they cleared protesters from a central roundabout where they had been camped out for weeks, a Reuters witness said.   

Protesters threw petrol bombs at police who advanced in thick lines from the Bahrain Financial Harbour, the country's main financial district to the Pearl roundabout, firing dozens of rounds of teargas.

Helicopters buzzed overhead as youths ran towards police in waves and scattered when new rounds of teargas hit.

Young men drove at high speed in their cars as they left the area, knocking down a policeman who was evacuated by ambulance, a Reuters witness who saw the incident reported.

There were no immediate reports of other injuries and the roundabout had been mostly cleared within two hours.

The security operation came a day after Bahrain's king declared martial law as his government struggled to quell an uprising that has drawn in troops from neighbour Saudi Arabia.   

It did not appear that Gulf forces invited in by the government for support were involved in the operation.
   
Thousands of protesters had been camped out at the Pearl roundabout, near the financial district, during weeks of protests. On Sunday, they overwhelmed police and blocked a main thoroughfare leading to the financial area.

Metal barricades and piles of rocks have blocked the main road since and most shops in Bahrain have been shut.

Three people, including one policeman, were killed, and more than 200 were wounded in various clashes on Tuesday.   

The unrest prompted Bahraini officials to issue stark warnings on Tuesday that martial law could mean imposing a curfew, evacuating areas and dispersing gatherings.

'In order for the situation to return to normal we have to establish order and security and ... stop the violations which have spread disturbances among the people of our dear country,' Interior Minister Sheikh Rashed Al-Khalifa said.   

As protesters fled, Bahraini security forces in light armoured personnel carriers began to clear the makeshift roadblocks. Refuse disposal trucks moved in to remove the debris and tents, some of which protesters set on fire as they left. -Reuters