A day after security forces assaulted protesters in a central square in defiance of U.S. calls for dialogue, authorities arrested at least six opposition leaders Thursday and accused them of inciting murder and destruction of property.

Opposition groups said the six were arrested as part of an apparently widening crackdown on protests by members of Bahrain’s Shiite majority, who harbor mounting grievances against the tiny Persian Gulf state’s Sunni monarchy.

In a statement read on Bahrain state television, the government did not identify the detainees or say how many were taken into custody. But opposition sources said six were detained, including one man who had been laying the groundwork for negotiations with the government over the political crisis that has convulsed this country for more than a month.

The government said those arrested were “leaders of the civil strife” who had “communicated with foreign countries,” Reuters news agency reported. The government charged that the men “incited killing of citizens and destruction of public and private property.”

The sweep came a day after security forces wiped out an opposition tent city in Pearl Square in the heart of the capital and declared a 4 p.m. to 4 a.m. curfew. The government said Thursday it would shorten the curfew by four hours in parts of the capital, starting it at 8 p.m.

Amid the crackdown, at least two small government hospitals remained closed Thursday despite a need for medical care.

The gates to Naim Hospital were chained shut Thursday morning, and Jidhafs Maternity Hospital was eerily empty, with no doctors, few office workers and dozens of empty beds in the main entrance halls.

Workers said the Ministry of Health ordered them closed Wednesday afternoon and that most patients went to private homes, transported by car.

At the private International Hospital of Bahrain, workers showed a shattered window that they said had been shot out Wednesday by police.

After the assault, trails of acrid black smoke floated over Manama as dumpsters and tires were set alight across the city.

Bahrain launched the crackdown despite U.S. insistence that dialogue, not violence, was the only way to resolve the protests, and it drew an unusually sharp rebuke from Secretary of State Hillary Rodham Clinton, who was visiting the Middle East.

“They are on the wrong track,” she told reporters in Cairo. “There is no security answer to this,” she added, referring to the protesters’ demands, “and the sooner they get back to the negotiating table and start trying to answer the legitimate needs of the people, the sooner there can be a resolution.”

The assault Wednesday, which left at least five people dead, was no more deadly than a nighttime raid on Pearl Square in February that killed at least four. But it appeared in some ways to deliver a more definitive blow to the protesters, followed not just by the curfew and arrests but by tougher government rhetoric and a heavier troop presence on the streets.

The move also came after two days in which dozens of tanks and hundreds of troop carriers, armored personnel carriers and water tankers streamed over a causeway connecting Bahrain to Saudi Arabia. The Saudi government has said it will do whatever it takes to ensure the survival of the al-Khalifa monarchy, which has ruled Bahrain for more than two centuries.

Saudi troops and forces from other Persian Gulf countries did not appear to take part in Wednesday’s action, but the decision to clear the square highlighted a profound sectarian divide in the region. Iranian President Mahmoud Ahmadinejad described the assault as “foul and doomed,” according to Iran’s state-run news agency.

On Thursday, Iranian state media reported that Ahmadinejad has recalled Iran’s ambassador to Bahrain.

In Iraq, Grand Ayatollah Ali Sistani, a highly influential Shiite cleric, called on Bahrain’s government to stop using violence against its citizens. Sistani rarely comments on politics, and his words hold great weight for Shiites in the region.

The mostly Shiite protesters in Bahrain have been calling for democratic reforms and an end to what they say is systematic discrimination against them by the Sunni monarchy. Shiites make up about 70 percent of the population in Bahrain, according to most estimates, but constitute a minority in a parliament that is largely powerless anyway.

Saudi Arabia has voiced concern that if Bahrain is taken over by Shiites, the country would become a satellite state of Iran. But the crackdown may only increase protesters’ sympathy for the Shiite-ruled country, some observers said Wednesday.

“For the Saudis to be here is a challenge to the Iranians,” said Jasim Husain, a member of the main Shiite opposition party, al-Wefaq. “This is something we wanted to avoid.”

Protesters have strenuously maintained that they were not controlled by Iranians, an assertion largely supported by U.S. officials.

“This is a historical day in Bahrain,” Husain said. “There are a lot of injuries, for sure.” Husain said contact had been severed between opposition parties and the monarchy, which as recently as Sunday had appeared poised to begin negotiations with demonstrators.

One of the men involved in those early conversations was arrested Thursday morning. Ibrahim Sharif leads the leftist Waad political society, a secular group composed of mostly Sunni members, and has been a prominent member of the opposition. The party announced his arrest.

Also taken into custody Thursday was Abduljalil al-Singace, according to a source with direct knowledge of the situation who requested anonymity because of safety concerns. Singace, a leader in the hard-line Haq movement, was jailed last August but was freed in late February as part of concessions by the Khalifa royal family to protesters.

Others arrested were Hassan Mushaima of the Haq movement and Shiite activists Abdul Wahad Hussein and Hassan Hadad, according to Husain of al-Wefaq.

On Wednesday, Bahraini security forces massed near Pearl Square about 6:30 a.m., and police started sweeping through the area about an hour later, using tear gas and rubber bullets to disperse protesters, witnesses said.

The witnesses also reported hearing rifle fire in the area, and doctors at Manama’s main hospital said that they had treated several protesters who had been shot with live ammunition.

By 9 a.m., protesters had fled, on foot or by car, leaving the army in control of the square, where tear gas mixed with dark smoke rising from blazing tents. In the afternoon, state television broadcast footage of security forces walking through the square as large tanker trucks extinguished the flames. It was not clear who had set the fires, and each side blamed the other.

The Bahraini government said two police officers were run over and killed by protesters leaving the square, and the Reuters news agency reported that a third police officer later died, citing a hospital source. Doctors and human rights workers said there were at least two other deaths, but the number was difficult to independently verify.

In a statement, the government said that no live rounds were fired by police and that “the only fatalities” during the operation were two police officers who were “repeatedly run over by three vehicles containing protesters leaving the fringes of the scene.” It asserted that “no other injuries were recorded.”

The military said in a statement carried by the state-run Bahrain News Agency that the protesters were “outlaws who had terrorized citizens and residents and harmed the national economy.”

The square was far less crowded Wednesday morning than it had been in recent days, as many protesters had returned to their villages to protect their homes. Those who remained did not put up a stiff fight, witnesses said. The daylight action stood in contrast to the surprise nighttime attack in February.

The assault also appeared to encompass Salmaniya Medical Complex, Manama’s main hospital, where a doctor said police and soldiers took over the facility about 7 a.m.

“Our hospital is under siege,” said the doctor, who spoke on the condition of anonymity because he feared for his safety. “We have supplies. We have everything. But we don’t have the patients.” Just six to eight injured people arrived at the beginning of the fighting in Pearl Square, he said. In the afternoon, a single ambulance brought four seriously wounded people who had been hit by live fire, he said, adding that later, a dead man was brought in who had been shot in the back by a large-caliber gun.

A reporter tried to gain access to the Salmaniya Medical Complex but was stopped by military forces and told to turn back. Tanks blocked all the gates into the complex. Later in the day, roads surrounding the complex were reopened and dozens of troops, most carrying assault rifles and all wearing face masks, stood at the entrances. But most of the roads around the complex would have been impassable by ambulances anyway, with barricades of piled bricks, broken glass and overturned dumpsters, some in flames, blocking the way.

A group of five medical workers received permission in the afternoon to attempt to reach local health centers to treat wounded there. But as they left the gates of the medical center, police officers ordered them out of their ambulance, and they were beaten by police and army personnel, said one of the five, who spoke on the condition of anonymity. One of them suffered fractures in his hand, the medical worker said.

The health minister, a Shiite, resigned Wednesday along with 12 judges from Bahrain’s Sharia Court, and the housing minister, also a Shiite, announced that he was “boycotting” the government, according to the opposition al-Wasat newspaper.

Staff writer Joby Warrick in Cairo contributed to this report.