Tanks and armoured vehicles rolled into Bahrain's capital Thursday after riot police clashed with anti-government demonstrators camped out in Manama's main square.
Riot police armed with clubs and tear gas clashed with anti-government protesters during a pre-dawn raid on Pearl Square.
Hours after the attack, the military announced a ban on gatherings, saying on state TV that it had "key parts" of the capital under its control.
Security forces moved in on the square while many demonstrators — including women and children — were asleep. Police tore down tents, beat people and blasted some with teargas and rubber bullets.
Since mid-January, North Africa and the Middle East have been gripped by revolutionary fever.
"We yelled, 'We are peaceful! Peaceful!"' said protester Mahmoud Mansouri. "The women and children were attacked just like the rest of us."
Medical officials said that four people were killed in the clashes. The country's health minister later said that three people were killed and more than 200 were injured in the early morning.
Bahrain's parliament — minus opposition lawmakers who are staging a boycott — met in emergency session on Thursday. One pro-government member, Jamila Salman, broke into tears. A leader of the Shia opposition, Abdul-Jalil Khalil, said 18 lawmakers resigned to protest the killings.
Foreign Minister Khalid Al Khalifa said the violence that occurred during the raid on the protest encampment was "regrettable."
Speaking to reporters after meeting with his Gulf counterparts, Al Khalifa said the protesters were pushing the Sunni-ruled, Shia-majority nation to the "brink of the sectarian abyss" and "polarizing the country."
'Regime must go'
In the wake of Thursday's bloodshed, angry demonstrators chanted "the regime must go" and burned pictures of King Hamad bin Isa Al Khalifa outside the emergency ward at Salmaniyah hospital, the main state medical facility.
"We will take to the streets in larger numbers and honour our martyrs. The time for Al Khalifa has ended," said Makki Abu Taki, whose son was killed in the assault.
Bahrain: facts and figures
Geography: Located in the Middle East, Bahrain is a group of islands located to the east of Saudi Arabia. Its area is roughly 760 square kilometres.
Population: Estimates vary, but the World Bank says the country's population was 791,473 in 2009. Bahrain has a significant community of expatriates who are in the country to work.
Languages: Arabic (official), English, Farsi, Urdu.
Religion: Islam is the country's official religion. The country's ruling family is Sunni but the majority of Bahrainis are Shia.
Ruler: King Hamad bin Isa Al-Khalifa has ruled the kingdom since 1999.
Head of government: Prime Minister Khalifa bin Salman, who was appointed by the king in 1971.
Government: Bahrain has a constitutional hereditary monarchy, which means the king, as chief of state, is guided by a constitution, but his rights, duties and responsibilities are spelled out in written law or by custom.
Economy: The financial sector is its largest contributor to GDP, at 30 per cent, the U.S. State Department says.
Martin Chulov, a correspondent with the Guardian newspaper, said the security forces are "very much in control" of Manama at the moment.
"Downtown is completely empty, tanks, armoured vehicles, police are manning all access points to Pearl Square."
Chulov said that the protesters who were forced out of the square during the raid re-grouped at a nearby hospital, where they planned massive demonstrations for Friday.
"There were several running clashes this morning. Overall, it's a very tense situation in central Bahrain," he said.
The Interior Ministry sent text messages to Bahrainis using cellphones, warning them to stay off the streets. Banks and other key institutions did not open, and workers stayed home, unable or afraid to pass through checkpoints to get to their jobs.
The protesters' demands have two main objectives: force the ruling Sunni monarchy to give up its control over top government posts and all critical decisions, and address deep grievances held by the country's majority Shias who claim they face systematic discrimination and are effectively blocked from key roles in public service and the military.
But among Bahrain's rulers, the prospect of a prolonged crisis raised fears of a potential flashpoint between Iran and its Arab rivals in the Gulf.
Bahrain's ruling Sunni dynasty is closely allied to Saudi Arabia and the other Arab regimes in the Gulf. Shia hard-liners in Iran have often expressed kinship and support for Bahrain's Shia majority, which accounts for roughly 70 per cent of the island nation's citizens.
Chulov said the mood on the street indicated there is "very definitely a mood for change."
"There's a lot of rage, there's a lot of motivation and there's a lot of incentive to actually bring about change."
Calls for restraint
The Obama administration expressed alarm over the violent crackdown.
"The United States is very concerned by recent violence surrounding protests in Bahrain," U.S. State Department spokesman PJ Crowley said in a statement. "We also call on all parties to exercise restraint and refrain from violence."
The European Union's chief diplomat also called for restraint. Catherine Ashton released a statement saying she "deplored" the deaths in Bahrain.
She also urged the government to respect the "fundament rights of their citizens" and engage in dialogue.
Meanwhile, U.S. Defence Secretary Robert Gates and Secretary of State Hillary Rodham Clinton both spoke with their counterparts in Bahrain, which is a key U.S. ally.
It is also home to the U.S. Navy's 5th fleet, which operates at least one aircraft carrier in the Gulf region at all times, along with an "amphibious ready group" of ships with Marines aboard.
The 5th Fleet's presence is central to a longstanding U.S. commitment to ensure the free flow of oil through the Gulf, while keeping an eye on a hostile Iran and seeking to deter piracy in the region.
Clinton told reporters she "directly conveyed our deep concerns about the actions of the security forces" there. She noted that there would be funerals and prayer meetings on Friday and said she had expressed hope they "not be marred by violence."