The violence came as the government announced a state of emergency and called in Saudi troops to keep order.

US Secretary of State Hillary Clinton has appealed for calm and restraint.

"The use of force and violence from any source will only worsen the situation," she told reporters during a visit to the Egyptian capital, Cairo.

A diplomatic row has also flared over the issue, with Bahrain recalling its ambassador in Tehran and complaining of "blatant interference" in its affair because Iran had condemned the arrival of foreign forces.

Earlier, Bahrain's King Hamad Bin Isa Al Khalifa decreed that there would be a three-month state of emergency to help restore order.

The head of the armed forces had been authorised to take all measures to "protect the safety of the country and its citizens", the announcement said.

On Monday, more than 1,000 troops from the Gulf Co-operation Council rolled into the country at the king's request, flashing victory signs.

Thousands of Bahrainis marched on the Saudi embassy in the capital, Manama, on Tuesday to protest against the intervention.

After the declaration of the state of emergency, many protesters set up barricades to protect themselves against the security forces. Young men, some wearing masks and carrying sticks, stood guard.

Later, there were violent clashes in several mainly Shia areas. In the village of Sitra, 15km (10 miles) south of Manama, police officers armed with shotguns fired on residents, a witness told the BBC.

Medics said more than 200 people had been injured in the clashes, and that two men - one Bahraini and the other Bangladeshi - had been killed.