Concerned that events in Bahrain could destabilise the entire region, Gulf foreign ministers met in Manama later on Thursday and expressed their "total support for Bahrain in the areas of politics, economy, security and defence."
In Washington, US President Barack Obama expressed opposition to the use of force, while Secretary of State Hilary Clinton said those who did use it should be held accountable.
Witnesses said riot police stormed through Pearl Square, the epicentre of pro-democracy protests that have shaken the Gulf island state, in the early hours of Thursday firing hollow-point bullets, rubber bullets and tear gas, sending hundreds of protesters fleeing.
"The health ministry has counted three dead and around 195 wounded," Health Minister Faisal al-Hamr was quoted by the official BNA news agency as saying.
Hamr said most of those injured had returned home, but that 43 people were still being treated, including one whom doctors were trying to resuscitate.
Opposition members and witnesses said the police raid was launched without warning at around 3:00 am (midnight GMT).
"They attacked the square, where hundreds of people were spending the night in tents," said one witness, 37-year-old Fadel Ahmad.
Relatives of the victims gathered outside the hospital, angry and weeping. In the course of the day, hundreds of people joined them, chanting "Death to Al-Khalifa" -- referring to the royal family.
Others queued to donate blood.
Thursday's deaths brought the total to five since Monday, when authorities began cracking down on protesters demanding political and social reform.
Security forces deployed across Manama, with armed police blocking roads leading to the square and setting up checkpoints in other streets, causing heavy traffic congestion.
Witnesses said dozens of armoured military vehicles were parked near Pearl Square as the military warned of "strict measures" to restore security in Bahrain, where clashes this week have left a total of six people dead.
Bahrain's defence forces will "take all strict and preventive measures to restore security and public order," a defence ministry spokesman said in a statement.
The ministry also urged people to "refrain from gathering in vital areas."
Interior ministry spokesman General Tarek al-Hassan said in an earlier statement that police had had no option but to raid the square.
"The security forces evacuated Pearl Square ... after having exhausted all chance of dialogue," Hassan said, as quoted by the official news agency BNA.
"Some left the place of their own accord, while others refused to submit to the law, which required an intervention to disperse them," he said.
Bahraini Foreign Minister Khaled bin Ahmed al-Thani told a press conference on Thursday evening that the police intervention was justified to prevent "a sectarian conflict and an economic crisis."
Thousands of demonstrators, inspired by uprisings in Tunisia and Egypt, had been occupying the square since Tuesday, after police killed two young Shiite demonstrators during anti-government protests.
Bahrain's opposition demanded the resignation of Prime Minister Khalifa bin Salman al-Khalifa's government in the wake of the raid, a Shiite opposition bloc said.
"The opposition groups, including Al-Wefaq, have issued a statement demanding the government resign and calling for the formation of a new government to investigate this crime," said Al-Wefaq bloc's leader, Ali Salman.
"We have decided to completely pull out from parliament," added Salman, whose bloc holds 18 seats in the country's 40-member elected house.
A US official said in Washington that Secretary of State Hillary Clinton had spoken to Sheikh Khaled on Thursday, urging Bahrain to show restraint over protests.
"She expressed deep concern about recent events and urged restraint moving forward. They discussed political and economic reform efforts to respond to the citizens of Bahrain," the State Department official told AFP.
She also said Bahrain must "hold accountable" those who use excessive force.
And Obama's spokesman Jay Carney said the president's view is that "we oppose the use of violence by the government of Bahrain."
At the same time, the Pentagon said US Defense Secretary Robert Gates discussed with the deputy commander of the Bahraini military by telephone the "current security situation."
Bahrain is home to the US Navy's Fifth Fleet.
British Foreign Secretary William Hague also called on Bahrain to exercise restraint, and Alistair Burt, a junior foreign minister with responsibility for the Middle East said London was reviewing decisions to export arms to the country.
And Iran, an overwhelmingly Shiite country, called on Bahrain to exercise restraint and heed the demands of its people.
For their part, the GCC foreign ministers statement also said that "our security is a collective responsibility and there is no question of accepting foreign interference."
The GCC groups Bahrain with Oman, Qatar, Saudi Arabia and the United Arab Emirates.
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