Soldiers and riot police in Bahrain overran a protesters' camp, imposed a 12-hour curfew and choked off movement nationwide Wednesday.
Witnesses described helicopters firing on homes in a hunt for Shiites and attacking doctors treating the wounded, while the Sunni-led government called the demonstrators "outlaws" for demanding an end to the monarchy.
The nation that once led the Middle East in entrepreneurial openness went into lockdown, its government propped up by troops from Sunni Gulf neighbors fearful for their own rule and the spread of Shiite Iran's influence.
The unrest that began last month increasingly looks like a sectarian showdown. The country's Sunni leaders are desperate to hold power, and majority Shiites want more rights and an end to the monarchy.
Wednesday's assault began in Pearl Square, the center of the uprising inspired by Arab revolts in Egypt and Tunisia. But the violence that left at least five people dead on Wednesday did not stop in the capital.
Doctors at the country's main hospital said their facility was taken over by security forces, blocking physicians from leaving or treating the wounded on site.
"There are many people injured, but we can't bring them to the hospital because of the travel restrictions, and doctors can't come to us," said Ali Marsouk, a resident of the Shiite village of Sitra, who said helicopters fired on homes in a three-hour attack.
Rania Ali, a resident, said police were charging after Shiites as they sought shelter. "I saw them chasing Shiites like they were hunting," said Ali, a Sunni whose husband is Shiite.
The Salmaniya hospital complex has become a political hot spot. Its mostly Shiite personnel are seen by authorities as possible protest sympathizers. The staff claim they must treat all who need care.
There have been moments of open anger among hospital staff. As overwhelmed teams treated the injured from Tuesday's clashes, many called for toppling the monarchy.
"We are under siege," said Nihad el-Shirawi, an intensive care doctor who said she had been working for 48 hours. "We cannot leave, and those on-call cannot come in."
Hospital officials, speaking on condition of anonymity, said they took in 107 injured from Wednesday's violence. Nine were in critical condition, they said.
President Obama called King Abdullah of Saudi Arabia and King Hamad bin Isa Al Khalifa of Bahrain, which hosts the U.S. Navy's 5th Fleet, to express deep concern over the violence. White House spokesman Jay Carney said Obama stressed the need for "maximum restraint."
Bahrain's sectarian clash is increasingly viewed as an extension of the region's rivalries between the Gulf Arab leaders and Iran. The United States, too, is being pulled deeply into the Bahrain conflict because of its naval base -- the Pentagon's main Gulf counterweight to Iran.
Iranian President Mahmoud Ahmadinejad on Wednesday denounced the crackdown and the presence of the Saudi-led force.
"How is it possible to stop waves of humanity with military force?" Ahmadinejad said.
In developments elsewhere in the Mideast:
Government supporters in Yemen armed with sticks, knives and guns attacked thousands of protesters, wounding hundreds in an increasingly violent crackdown on demonstrations calling for Yemen's longtime president to step down.
After nearly a month of protests, about 50 demonstrators have been killed in the country, said Amal al-Bashi, of the Yemen Center for Human Rights. Protesters demand that President Ali Abdullah Saleh leave office.
In Damascus, plainclothes security officers armed with batons dispersed about 100 protesters in the Syrian capital, beating some and detaining at least 30 people, witnesses and rights groups said. It was the second time Syrian forces have violently dispersed a small protest this week.