Security forces overran a central square of Bahrain’s capital Wednesday, firing tear gas and rubber bullets and clearing out an encampment of protesters a day after a state of emergency was declared.

Shortly after dawn, black smoke could be seen rising from Pearl Square, the epicenter of protests over the last month. Helicopters hovered overhead, and much of the central city was on lockdown. A doctor at the main hospital said police and army forces had taken control of the complex and would not allow injured people waiting outside to come in.

In the afternoon, the military announced a curfew that would ban access to Pearl Square and most of downtown from 4 p.m. Wednesday (9 a.m. in Washington) until 4 a.m. Thursday.

State television broadcast live footage of security forces walking through the square as fires raged among the tents. Large tanker trucks with riot equipment on their fronts were putting out the blazes. It was not clear who had set the fires, but security forces had controlled the area for more than six hours.

Eyewitness accounts on Twitter described riot police, security forces and many injuries. Two protesters were killed, according to a human rights worker. State television said two policemen died when they were struck by a vehicle. There was no other word on casualties.

The United States condemned Bahrain’s assault on the protesters and called for political reform. “We object to excessive force and violence against demonstrators,” the State Department said Wednesday, adding that it conveyed U.S. concerns to Bahrain’s government. “We continue to believe the solution is credible political reform, not security crackdowns that threaten to exacerbate the situation.”

In a statement, the government insisted that no live rounds were used by the 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 statement issued by the Bahrain Information Affairs Authority also charged that security forces “came under gunfire” on their way to Pearl Square and were attacked by “around 250 saboteurs” who hurled molotov cocktails from an overpass next to the roundabout.

Opposition figures and medical personnel told a far different story.

“There are a lot of injuries for sure,” said Jasim Husain, a member of al-Wefaq, the largest opposition political party. “This is a historical day in Bahrain.”

A doctor inside the Salmaniya Medical Complex, Manama’s principal hospital, said that police and army forces took over the complex around 7 a.m.

“Our hospital is under siege,” said the doctor, who asked for anonymity because he feared for his safety. “There are lots of injured people outside the hospital entrance. They are not allowed inside the hospital, and we are not allowed to go out and bring them in,” he said.

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, and soldiers were not allowing any cars onto the streets surrounding the center. The Salmaniya doctor said that there were no military forces inside the complex itself, just police officers, who were not allowing doctors in or out of the hospital. Ambulances also were not allowed in or out of the hospital, the doctor said.

“We are waiting, we are waiting for injured to come. We really need help,” he said. “We have supplies, we have everything, but we don’t have the patients.” Just six to eight injured people came at the beginning of the fighting in Pearl Square, he said.

The doctors were organizing themselves into teams to go to local health centers and mosques, where injured people had gathered, but they had not received permission from security forces to leave the complex, the Salmaniya doctor said.

Later, shortly before a 4 p.m. curfew started, four police ambulances, sirens blaring and blue lights flashing, could be seen pulling away from the Salmaniya Medical Complex, apparently driven by security forces wearing white ski masks. A tank was still stationed in front of the main gate to the center, but a civilian car was being let out.

Dozens of soldiers, most carrying assault rifles and all wearing ski masks, were posted around the perimeter of the hospital. Traffic was allowed on at least some of the streets around the hospital, a change from earlier in the day.

Inside Salmaniya, a doctor said by telephone that a small team had been allowed to go to local health centers to treat patients there, more than seven hours after the initial assault on Pearl Square. He said a small number of seriously wounded people from private hospitals had been allowed into the larger, better equipped Salmaniya.

But as of 4:00 p.m., the military was no longer allowing entry or exit, he said.

The doctor said he had treated numerous patients with wounds from live ammunition — and he excused himself to go treat another one.

Roadblocks made traversing the streets of downtown Manama virtually impossible, as cars had to navigate a tight obstacle course between piles of bricks, overturned dumpsters and broken glass, all put there to obstruct passage.

Bahrain’s king declared a state of emergency Tuesday, imposing a curfew, banning rallies and handing broad powers to a military bolstered by the fresh arrival of dozens of tanks sent by other Persian Gulf monarchies.

On Monday and Tuesday, thousands of security forces, many of them Saudi, poured into Bahrain over a causeway that connects the tiny island nation to Saudi Arabia’s oil-rich Eastern Province. It was not immediately clear whether the foreign security forces had been involved in Wednesday’s actions.

The arrival of armor and the dispatch of Saudi troops reinforced a new hard line adopted by Bahrain’s ruling family in a bid to quash a month-long protest that continued Tuesday, with human rights advocates reporting hundreds injured.

The Bahrain Defense Force warned ominously Tuesday that “certain locations will be evacuated” under the state of emergency, according to the state news agency, which called it a “crackdown on lawbreakers.” Protesters had occupied Pearl Square in central Manama for more than three weeks, and their new grip on the downtown area had expanded in recent days to include checkpoints and barriers that have brought the country’s economy to a near halt.

Pearl Square was less crowded Tuesday evening than it had been in recent weeks, because many people had gone to their home villages to defend them against attacks by security forces and roaming bands of pro-government civilians.

On Wednesday morning, state television broadcast live shots of the square showing that protesters had been cleared out and many tents had been leveled. Several large fires were burning at the bases of some palm trees, and some tents also appeared to be on fire. A large security force could be seen massed at the perimeter.

A streaming red ticker underneath the broadcast said that “the saboteurs have pulled out of the GCC Roundabout after the arrival of the police.” Officials started referring to Pearl Roundabout as the GCC Roundabout three days ago.

Elsewhere in the capital, fires raged in dumpsters that had been set up as roadblocks to restrict access to the inner core of the city. Thick black smoke could be seen rising from a park near Pearl Square, and an acrid rubber smell permeated the air.

Armored vehicles blocked a major intersection close to the square, and soldiers with rifles checked identification and turned people away. More than a dozen cars with smashed windows and flat tires were scattered across the intersection. It was unclear when the cars had been damaged and by whom.

Military forces barricaded many intersections, but some entrances to the city center were blocked by men in civilian clothes, armed with long sticks. A main access road to the airport, which is on an island connected to the main city of Manama by a bridge, was blocked by a tan tank and two Humvees. Drivers said they were not able to get to the airport.

Tuesday’s announcement of a “State of National Safety’’ was broadcast on state television and was described by Bahraini officials as a step below martial law. The officials said it would last three months and would target what the Information Ministry called “increased lawlessness jeopardizing the lives of citizens.”

Saudi Arabia, which like Bahrain has a Sunni monarchy, has feared that Bahrain’s mostly Shiite protesters could fall in thrall to Iran were they to gain power, and it has vowed to do anything it takes to support Bahrain’s al-Khalifa royal family.

There were signs of fresh tension between Iran and its neighbors over the Saudi intervention. A spokesman for Iran’s Foreign Ministry said Tuesday that “the presence of foreign forces and interference in Bahrain’s internal affairs is unacceptable and will further complicate the issue.” Bahrain recalled its ambassador from Tehran.

At least two people died in clashes around Bahrain on Tuesday, according to the state-run television station and human rights advocates. There were conflicting accounts about the nationalities of the victims.

Witnesses watching the causeway between Saudi Arabia and Bahrain said that about 100 army trucks and dozens of tanks on transport vehicles arrived Tuesday, along with more armored personnel carriers, supplementing the more than 1,000 Saudi troops and 500 police officers from the United Arab Emirates who arrived Monday. Bahrain’s own defense force includes only about 9,000 personnel. It was not immediately clear which Gulf countries the new military forces had come from.

Protesters had vowed to hold their ground, and a member of the main opposition political group said that chances of negotiation were slim as long as foreign troops were in the country.

“There’s no talk about dialogue,” said Husain of al-Wefaq. “The topics have changed in the last 24 hours.”

Still, Husain said, violence would not solve the issues and “eventually there’s no choice” but for negotiations.

He said that he was not aware of any contact between the opposition groups and the government since the troops arrived Monday.

U.S. Secretary of State Hillary Rodham Clinton said she had told Saudi Foreign Minister Saud al-Faisal in a conversation Tuesday that all sides “must take steps now to negotiate toward a political resolution.” While the United States recognized that Bahrain has “the right to ask for assistance,” Clinton said in a news conference in Cairo, “security challenges cannot be a substitute for a political resolution.”

“I’m not going to characterize their actions,” Clinton said of the Saudis. But “as long they are moving in” to Bahrain, she said, “they, along with everyone else, need to be promoting the dialogue between the two parties.”

Staff writers William Branigin and Karen DeYoung contributed from Washington.