MANAMA, Bahrain (AP) -- Troops and tanks locked down the capital of this tiny Gulf kingdom after riot police swinging clubs and firing tear gas smashed into demonstrators, many of them sleeping, in a pre-dawn assault Thursday that uprooted their protest camp demanding political change. Medical officials said four people were killed.

Hours after the attack on Manama's main Pearl Square, the military announced a ban on gatherings, saying on state TV that it had "key parts" of the capital under its control.

After several days of holding back, the island nation's Sunni rulers unleashed a heavy crackdown, trying to stamp out the first anti-government upheaval to reach the Arab states of the Gulf since the uprisings in Tunisia and Egypt. In the surprise assault, police tore down protesters' tents, beating men and women inside and blasting some with shotgun sprays of birdshot.

It was a sign of how deeply the Sunni monarchy - and other Arab regimes in the Gulf - fear the repercussions of a prolonged wave of protests, led by members of the country's Shiite majority but also joined by growing numbers of discontented Sunnis.

Tiny Bahrain is a pillar of Washington's military framework in the region. It hosts the U.S. Navy's 5th Fleet, which is a critical counterbalance to Iran. Bahrain's rulers and their Arab allies depict any sign of unrest among their Shiite populations as a move by neighboring Shiite-majority Iran to expand its clout in the region.

But the assault may only further enrage protesters, who before the attack had called for large rallies Friday. In the wake of the 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 are even angrier now. They think they can clamp down on us, but they have made us angrier," Makki Abu Taki, whose son was killed in the assault, shouted in the hospital morgue. "We will take to the streets in larger numbers and honor our martyrs. The time for Al Khalifa has ended."

The Obama administration expressed alarm over the violent crackdown. Secretary of State Hillary Rodham Clinton called Bahrain's foreign minister to register Washington's "deep concern" and urge restraint. Similar criticism came from Britain and the European Union.

Salmaniyah hospital was thrown into chaos by a stream of dozens of wounded from Pearl Square, brought in by ambulances and private cars. At least one of the dead was peppered with bloody holes from pellets fired from police shotguns. Nurses rushed in men and women on stretchers, their heads bleeding, arms in casts, faces bruised. At the entrance, women wrapped in black robes embraced each other and wept.

The capital Manama was effectively shut down Thursday. For the first time in the crisis, tanks rolled into the streets and military checkpoints were set up as army patrols circulated. The Interior Ministry warned Bahrainis to stay off the streets. Banks and other key institutions did not open, and workers stayed home, unable or to afraid to pass through checkpoints to get to their jobs.

Barbed wire and police cars with flashing blue lights encircled Pearl Square, the site of anti-government rallies since Monday. The square was turned into a field of flattened tents and the strewn belongings of the protesters who had camped there - pieces of clothing and boxes of food.

Banners lay trampled on the ground, littered with broken glass, tear gas canisters and debris. A body covered in a white sheet lay in a pool of blood on the side of a road nearby.

Demonstrators had been camping out for days around the landmark square's 300-foot (90-meter) monument featuring a giant pearl, a testament to the island's pearl-diving past.

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 Shiites who make up 70 percent of Bahrain's 500,000 citizens but claim they face systematic discrimination and poverty and are effectively blocked from key roles in public service and the military.

Shiites have clashed with police before in protests over their complaints. But the growing numbers of Sunnis joining the latest protests have come as a surprise to authorities, said Simon Henderson, a Gulf specialist at the Washington Institute for Near East Policy.

"The Sunnis seem to increasingly dislike what is a very paternalistic government," he said, adding that the crackdown was "symptomatic" of Gulf nations' response to crises. "As far as the Gulf rulers are concerned there's only one proper way with this and that is: be tough and be tough early."

The assault came early Thursday with little warning, demonstrators said. Police surrounded the square and then quickly moved in. Some lined up on a bridge overhead, pumping down volleys of tear gas, as others waded into the camp, knocking down tents and swinging truncheons at those inside.

"We yelled, 'We are peaceful! Peaceful!'" said protester Mahmoud Mansouri. "The women and children were attacked just like the rest of us."

Dr. Sadek Al-Ikri, 44, said he was tending to sick protesters at a makeshift medical tent in the square when the police stormed in. He said he was tied up and severely beaten, then thrown on a bus with others.

"They were beating me so hard I could no longer see. There was so much blood running from my head," he said. "I was yelling, 'I'm a doctor. I'm a doctor.' But they didn't stop."

He said the police beating him spoke Urdu, the main language of Pakistan. A pillar of the protest demands is to end the Sunni regime's practice of giving citizenship to other Sunnis from around the region to try to offset the demographic strength of Shiites. Many of the new Bahrainis are given security posts.

Al-Ikri said he and others on the bus were left on a highway overpass, but the beatings didn't stop. Eventually, the doctor said he fainted but could hear another police official say in Arabic: "Stop beating him. He's dead. We should just leave him here."

Many families were separated in the chaos. An Associated Press photographer saw police rounding up lost children and taking them into vehicles.

Hussein Abbas, 22, was awakened by a missed call on his cell phone from his wife, presumably trying to warn him about reports that police were preparing to move in.

"Then all of a sudden the square was filled with tear gas clouds. Our women were screaming. ... What kind of ruler does this to his people? There were women and children with us!"

ABC News said its correspondent, Miguel Marquez, was caught in the crowd and beaten by men with billy clubs, although he was not badly injured.

The violence killed four people, said hospital officials, speaking on condition of anonymity because they were not authorized to talk to the media.

Bahrain's parliament - minus opposition lawmakers who are staging a boycott - met in emergency session. One pro-government member, Jamila Salman, broke into tears. A leader of the Shiite opposition Abdul-Jalil Khalil said 18 lawmakers resigned to protest the killings.

Hours before police moved in, the mood in the makeshift tent city was festive and confident.

People sipped tea, ate donated food and smoked apple- and grape-flavored tobacco from water pipes. The men and women mainly sat separately - the women a sea of black in their traditional dress. Some youths wore the red-and-white Bahraini flag as a cape.

While the protests began as a cry for the country's Sunni monarchy to loosen its grip, the uprising's demands have steadily grown bolder. Many protesters called for the government to provide more jobs and better housing, free all political detainees and abolish the system that offers Bahraini citizenship to Sunnis from around the Middle East.

Increasingly, protesters also chanted slogans to wipe away the entire ruling dynasty that has led Bahrain for more than 200 years and is firmly backed by the Sunni sheiks and monarchs across the Gulf.

The stability of Bahrain's government is seen as crucial by its other allies in the Gulf, who - though they rarely say it in public - see Bahrain's Shiite majority as the weak link in their unity against Iranian influence.

Hard-liners in Iran have often expressed kinship and support for Bahrain's Shiites. But in Bahrain, the community staunchly denies being a tool of Tehran, saying their complaints are rooted in their country's unbalanced system.

Although Bahrain is sandwiched between OPEC heavyweights Saudi Arabia and Qatar, it has limited oil resources and depends heavily on its role as a regional financial hub and playground for Saudis, who can drive over a causeway to enjoy Bahrain's Western-style bars, hotels and beaches.

The unrest could threaten the opening next month of Formula One racing, one of the centerpieces of Bahrain's claims for international prestige. The GP2 Asia Series race, due to start Friday on the same circuit used by Formula One, was called off at the request of the Bahrain Motorsport Federation "due to force majeure," race organizers announced Thursday.

Social networking websites had been abuzz Wednesday with calls to press ahead with the protests. They were matched by insults from presumed government backers who called the demonstrators traitors and agents of Iran.

The protest movement's next move is unclear.

Before the attack on the square, protesters had called for major rallies after Friday prayers. The reported deaths, however, could become a fresh rallying point. Thousands of mourners had turned out for the funeral processions of two other people killed in the protests earlier in the week.

After prayers Wednesday evening, a Shiite imam in the square had urged Bahrain's youth not to back down.

"This square is a trust in your hands and so will you whittle away this trust or keep fast?" the imam said. "So be careful and be concerned for your country and remember that the regime will try to rip this country from your hand but if we must leave it in coffins then so be it!"

Across the city, government supporters in a caravan of cars waved national flags and displayed portraits of the king.

"Come join us!" they yelled into markets and along busy streets. "Show your loyalty."

Thousands of mourners turned out Wednesday for the funeral procession of 31-year-old Fadhel al-Matrook, one of two people killed Monday in the protests. Later, in Pearl Square, his father Salman pleaded with protesters not to give up.

"He is not only my son. He is the son of Bahrain, the son of this nation," he yelled. "His blood shouldn't be wasted."

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Brian Murphy in Dubai, United Arab Emirates, contributed to this report.