At least six people are reported dead and hundreds injured after security forces in Bahrain drove out pro-democracy protesters from the Pearl Roundabout in the capital, Manama.
A 12-hour curfew came into force at 4pm in areas of the city including the roundabout, the Bahrain Financial Harbour, and several other buildings which have recently been targets of protests.
By then, most of the area had been cleared after troops backed by tanks and helicopters stormed the site - the focal point of weeks-long anti-government protests in the tiny kingdom - early on Wednesday, an Al Jazeera correspondent said.
Multiple explosions were heard and smoke was seen billowing over central Manama.
Hospital sources said three protesters had been killed and hundreds of others injured in the offensive, the Reuters news agency reported. Three policemen were also reported dead.
The crackdown drew international criticism, with Hillary Clinton, the US secretary of state, warning that Bahrain and its Gulf Co-operation Council (GCC) allies were "on the wrong track." Both Bahrain and Saudi Arabia, which has sent soldiers to Bahrain to support the government, are key US allies in the region.
Iran and Iraq also condemned the violence.
Our correspondent said police backed by the military attacked the protesters from all sides and used tear gas canisters to disperse the crowd.
Protesters, intimidated by the numbers of security forces, retreated from the roundabout, he said. By 5pm the area was quiet, although a few people remained on the streets. A helicopter circled overhead.
Doctors and witnesses at the city's main Salmaniya hospital have told Al Jazeera that they have been "besieged" by security forces.
"We are besieged here since the morning. No one can get in or out of the hospital as a result of the conflict at the Roundabout. Bahraini army, police and Saudi security are using tanks to prevent people from entering," Abdul Mohamed, an eyewitness, told Al Jazeera.
"There are also other forces I cannot identify in civilian clothing ... There is a large number of injured, over 400 people, including women and children."
Dr Nehad Shirawi, the head of the intensive care unit at the hospital told Al Jazeera via telephone: "We are scared to get out of the hospital. We don't think it's safe to go out and we don't know what to do.
"We are physically and mentally exhausted and I don't think we'll be able to continue to attend to patients in this way. We need to be replaced by other doctors so we can go home and rest."
Bahrain's youth movement had called for a mass demonstration on Wednesday afternoon but it was unclear whether protesters planned to regroup elsewhere in the city.
Bahrain's main opposition Wefaq party has called off 4protests, saying it is too dangerous to continue. There are fears that a small gathering could result in a high level of casualties, our correspondent said.
Wefaq has advised people since this morning to avoid confrontation with security forces and to remain peaceful," a Wefaq official told Reuters.
Ali Al Aswad, a Wefaq member, told Al Jazeera that the government used Apache helicopters to shoot at peaceful protesters.
He said the situation was very bad and Bahrain was heading towards a disaster. "The security forces are killing the people, we call upon UN to help us," Aswad said.
State of emergency
The move by the security forces came a day after a state of emergency was declared on the island and at least two people were killed in clashes in the Shia suburb of Sitra outside Manama.
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Hundreds of Saudi-led troops entered Bahrain on Monday as part of a Gulf Co-operation Council (GCC) initiative to help protect government facilities there amid an escalation in the protests against the government.
The United Arab Emirates also sent about 500 police to Bahrain, according to Abdullah bin Zayed Al-Nahyan, the Emirati foreign minister. Qatar, meanwhile, did not rule out the possibility of its troops joining the force.
It was not immediately clear if Wednesday's crackdown involved Saudi or GCC security forces.
Syed Al Alawi, a witness, told Al Jazeera that troops were surrounding the Salmania hospital and not allowing doctors and nurses to enter.
Calling for help, Alawi said: "The GCC troops are for fighting against foreign forces, instead they are targeting the people of Bahrain. What's our fault, we are asking for our legitimate rights."
At least 500 protesters have been camping at the Pearl Roundabout in central Manama as part of their demonstration.
The small kingdom with a dominant Shia majority has been swept by protests over the last several weeks. The protesters, alleging discrimination and lack of rights, are seeking political reforms.
Hillary Clinton, speaking in Egypt, said Bahrainis must "take steps now" towards a political resolution and said there was "no security answer" to the crisis.
"We have also made that very clear to our Gulf partners who are part of the Gulf Cooperation Council, four of whose members have sent troops to support the Bahraini government. They are on the wrong track," Clinton told the BCS television network.
Iran, meanwhile, has warned against "foreign interference".
"The peaceful demonstrations in Bahrain are among the domestic issues of this country, and creating an atmosphere of fear and using other countries' military forces to oppress these demands is not the solution," Hossein Amir Abdollahian, an official from the Iranian foreign ministry, was reported by Iran's semi-official Fars news agency as saying.
In Iraq, Prime Minister Nuri al-Maliki has spoken out against military intervention by Bahrain's Gulf allies.
"This will contribute towards complicating the situation in the region in a way that instead of solving it could lead to inflaming sectarian tension," he said in a statement on Wednesday.
Grand Ayotollah Ali al-Sistani, Iraq's top-ranked Shia cleric, who rarely makes statements regarding politics, has also weighed in, calling on Bahraini authorities to "stop using violence against unarmed citizens", Hamed al-Khafaf, his spokesman, said.
Meanwhile, in Baghdad, several thousand protesters gathered in cleric Moqtada al-Sadr's stronghold of Sadr City waving Bahraini and Iraqi flags. Basheer al-Najafi, another senior Shia cleric in Najaf, also condemned the Bahraini crackdown.