MANAMA – Bahrain's army deployed across Manama Thursday and vowed "strict measures" to restore order after a violent police raid on anti-regime protesters killed four, wounded scores and enraged the opposition.

Protesters gathered during the day outside a hospital where the wounded are being treated to chant anti-regime slogans, while the largest Shiite opposition bloc said it was quitting parliament and called on the government to resign.

Concerned that events in Bahrain could destabilise the entire region, Arab foreign ministers of the Gulf monarchies were to hold an emergency meeting later Thursday in Manama, Bahrain's foreign ministry said.

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.

Up to 95 protesters were wounded in the operation which was launched without warning at around 3.00 am (midnight GMT), opposition members and witnesses said.

"They attacked the square, where hundreds of people were spending the night in tents," said one witness, 37-year-old Fadel Ahmad.

At the city's main Salmaniya hospital, medical staff were overwhelmed as ambulances and private cars were still ferrying in the injured more than three hours after the assault began.

Relatives of the victims gathered outside the hospital, angry and weeping and while 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.

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," in Bahrain.

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.

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 Bahrain foreign ministry statement said the Gulf Cooperation Council (GCC) states at their meeting on Thursday are "expected to announce their support for the (Bahraini) government in security, defence and politically."

The GCC groups Bahrain with Oman, Qatar, Saudi Arabia and the United Arab Emirates.

A US official said in Washington that US Secretary of State Hillary Clinton had spoken Thursday to Bahraini Foreign Minister Sheikh Khaled bin Ahmad al-Khalifa, 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.

British Foreign Secretary William Hague also called on Bahrain to exercise restraint, while European Union foreign policy chief Catherine Ashton called on the authorities to respect the right of citizens to protest peacefully.

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