Sana, Yemen — Yemen’s pro-democracy protests exploded into violence on Friday, as government supporters opened fire on demonstrators in this capital, killing at least 45 people and wounding more than 200. The bloodshed failed to disperse the angry throng of tens of thousands of protesters, the largest seen so far in a month of demonstrations calling for the ouster of President Ali Abdullah Saleh.

Mr. Saleh declared a state of emergency shortly after the shootings, denying that security forces had been involved and promising a full investigation. The state news agency said the state of emergency would last 30 days.

The shootings seemed certain to provoke more violence in Yemen’s tribal society, and analysts said they could further weaken Mr. Saleh. Although the United States has voiced sympathy for pro-democracy protesters here and elsewhere in the Arab world, it has special concerns about the stability of Yemen, a strife-torn country that is home to one of Al Qaeda’s most active branches and has been an American ally, and a major recipient of military aid, in the fight against terrorism.

Protesters have been killed here in recent weeks, but the violence on Friday dwarfed that of earlier clashes. It began almost immediately after the protesters’ noon prayers, conducted en masse in the street by thousands. As the protesters rose from prayer, government supporters in plain clothes opened fire from rooftops and windows on parts of the crowd, while security forces fired guns and a water cannon.

Some of the men in the protest raided buildings where gunmen had been seen, catching several men accused of being snipers, dragging them into the streets and beating them. At least one home appeared to have been set on fire. In the apartment of one suspect, protesters said they had found military uniforms and Defense Ministry identification.

But in the chaos of the day and given the fact that most Yemenis are armed, it was not clear exactly how the violence began or whether the men who fired on the crowd acted on their own or as proxies for Mr. Saleh’s security forces. If the government was responsible, it would appear to have taken up the same strategy that Libya and Bahrain followed this week, using overwhelming force against protesters.

“It seems like people saw what happened in Bahrain and thought you could do the same here,” said one high-ranking Yemeni official, who said he did not know who was responsible for the outbreak. “But in Yemen it is going to be very bad — a disaster. This will change everything, because the people killed have tribes.”

At a news conference in Sana, Mr. Saleh claimed that the clashes on Friday were between “citizens and demonstrators” and that “the police were not present and did not open fire.”

President Obama condemned violence in a written statement that called on President Saleh “to adhere to his public pledge to allow demonstrations to take place peacefully.” He added, “Those responsible for today’s violence must be held accountable.”

Friday’s violence came just two days after a team of mediators from Saudi Arabia and other Gulf countries arrived in Sana to try to broker a deal between the government and Yemen’s opposition coalition, which is dominated by an Islamist party.

In his comments on Friday, Mr. Saleh referred to the mediation efforts, saying, “I regret that this event may delay a process that could have saved lives.” But many of the protesters are not affiliated with any established party, and it is not clear that they would have been guided by the deal makers.

The death toll rose through the afternoon as some of the more than 200 people wounded by gunfire or rocks hurled by government supporters succumbed to their injuries, according to a doctor, Muhammed Rizq, and others at a makeshift hospital near the protest site. The majority of those killed had been shot in the head or neck, doctors said. Many of the wounded were, too, and were expected to die.

Despite the heavy toll, the protesters in Sana kept control of a lengthening portion of Ring Road, which stretches from Sana University to a central highway overpass, as the shooting appeared to halt in the middle of the afternoon.

The security forces that had massed at the protest’s south end then began to pull back into the city center, firing tear gas as hundreds of protesters gave chase, hurling rocks. People in apartments overlooking the action tossed onions down to the protesters for them to use to relieve the effects of the tear gas.

Before the shooting, the protest had swelled to tens of thousands of people and stretched for a mile from its center at Sana University, and a heavy cloud of black smoke hung over downtown as government supporters burned protesters’ tents. Once the shooting began, many moved north along Ring Road and away from the fighting, but a crowd of mostly tribal men from the outskirts of the capital stood firm. A man walked through the crowd with a microphone yelling: “Peaceful, peaceful! Don’t be afraid of the bullets!”

Then the shooting appeared to stop, and the security forces withdrew about a mile down the wet, rock-strewn road.

“Today is the worst day; this is a new Qaddafi,” said Khalil al-Zekry, who hunkered down in his video shop along the protest route.

A coalition of Yemeni opposition parties called the JMP issued a statement saying that “this horrendous massacre” would not “discourage our people from continuing the struggle.”

The group said that it held “Mr. Saleh and his family and everyone who participated fully responsible” and called on Yemeni military officers and soldiers to refuse to participate in violence against Yemeni citizens. A key factor in the success of protest movements in Tunisia and Egypt was the reluctance of those countries’ armies to turn their guns on civilians.

Politically, it seemed clear that Friday’s violence would harm the Yemeni president. “It’s not in Saleh’s interest at all to have people get shot,” said Charles Schmitz, a Yemen expert at Towson University. That fact deepened the mystery over the shootings; some blamed them on overzealous Saleh supporters or even a Saleh rival in the government.

But despite the risk of more violence and instability, there are positive signs in the recent turmoil, Mr. Schmitz added, including the emergence in protests of a political coalition broader and more representative than anything Yemen has seen in decades.

Before Friday, at least 40 protesters had been killed in weeks of demonstrations across the country. Most of those deaths occurred in the restive southern port city of Aden, where protests have focused on seceding from the nation rather forcing Mr. Saleh from power.

Demonstrators in the capital have stressed the peaceful nature of their protests. Still, one protester, Abdul-Ghani Soliman, said he was not surprised by the violence. “I actually expect more than this, because freedom requires martyrs,” said Mr. Soliman, an unemployed tribesman from outside Sana. “This will continue, and it will grow.”

* Laura Kasinof reported from Sana, and Robert F. Worth from Washington.