Government forces were reported to be shelling several neighborhoods of Damascus on Thursday, a day after a bombing that killed three senior military figures brought the bloodshed that has engulfed Syria to the heart of President Bashar al-Assad’s regime.

The blast, which targeted a mid-morning meeting of top security chiefs charged with suppressing Syria’s 16-month-old uprising, appeared to have been meticulously planned and executed, suggesting that rebels have managed to infiltrate Assad’s security apparatus in ways that call into question his regime’s capacity to survive.

The attack seemed to mark a pivotal moment in the revolt, the first time that the regime’s tightly controlled and fiercely loyal inner circle has been punctured since Syrians first took to the streets to demand reforms in March 2011 and then later took up arms. Among those killed was Assef Shawkat, the president’s brother-in-law, who was widely feared as one of the chief enforcers of the brutal tactics that have been used in the effort to quell the unrest.

Rumors flew — that the president himself was dead, that he had fled the capital in a helicopter, that his notoriously ruthless brother Maher had been killed and that Free Syrian Army rebels were closing in on the presidential palace. Although none appeared to be true, they contributed to a building sense of anticipation that many months of stalemate on battlefields around the country could be coming to an end.

“Right now we are on the cusp. The regime might be able to contain it, or things might unravel completely,” said Amr al-Azm, a history professor at Ohio’s Shawnee State University who is active in the Syrian opposition.

In Washington, U.S. officials said the attack offered a clear sign of the Assad regime’s vulnerability. But they expressed anxiety about the potential for increased chaos as the rebels try to press their advantage and the regime seeks to reassert its authority.

“This is a situation that is rapidly spinning out of control,” Defense Secretary Leon E. Panetta said at a Pentagon news briefing.

‘Beginning of the end’

The opposition’s hopes that Assad’s hold was slipping were tempered by fears of a backlash after the government vowed to retaliate against the “terrorists” responsible for the bombing, warning that ordinary Syrians nationwide would arm themselves to defend the regime. “This is the decisive battle not only in Damascus but in Syria as a whole,” Information Minister Omran Zoabi said on state television. “They are wrong to underestimate us.”

The streets of Damascus emptied, shops closed and residents stayed indoors, bracing for what many predicted could be a sharp escalation in violence. “I think what we are seeing today is the beginning of the end,” said Tariq Saleh of the Revolutionary Leadership Council of Damascus, who was speaking from the capital via Skype. “The scenario we fear is that the loyalists will launch revenge attacks on civilians before we witness the fall of the regime.”

He added: “We only hope it ends soon, with as little bloodshed as possible.”

Already, there were reports that vengeance was being sought, with residents of several Damascus neighborhoods claiming that pro-government militias known as shabiha were roaming the streets, armed with knives and guns and targeting civilians. There were also reports that several neighborhoods that had witnessed clashes between rebels and security forces in recent days were being attacked with artillery and rockets fired by helicopters. “There are tens of bodies on the streets,” Saleh said.

The Local Coordination Committees, an opposition group, said 36 people were killed in Damascus and 32 in the city’s suburbs, including 15 who died when helicopters opened fire on a funeral in the Saida Zeinab area south of the capital. They were among more than 130 killed nationwide, the LCC said.

The information was impossible to confirm independently, but activists posted videos of what appeared to be three bodies in the streets of the suburb of Qadam.

“The situation is getting worse by the second,” said a resident of another suburb, Qaboun, who was contacted via Skype and asked not to be identified because he fears for his safety. “Armed people are walking in the streets.”

People were fleeing some of the worst-hit neighborhoods, including the flash point Midan district. In the Shaghour neighborhood of the historic Old City, residents carrying knives and sticks were in the streets to defend against a threatened attack by similarly armed residents from a nearby Shiite neighborhood, but community leaders calmed the situation.

There were also widespread reports of accelerating defections from within the security forces around the country, although they also could not be confirmed. Abu Emad, an activist in Homs, said that at least 250 soldiers had abandoned their posts in the Old City, long a battleground between rebels and government forces. But he also said government forces based on the periphery of Homs had launched an intensified bombardment against the area.

The Britain-based Syrian Observatory for Human Rights reported shelling in a number of areas Thursday, as residents fled parts of the Mezzeh neighborhood after troops surrounded it and clashed with local rebels, daminging one helicopter and disabling three military vehicles, according to the Associated Press.

Major blow to regime

Those killed in the bombing included Defense Minister Dawoud Rajha; Hassan Turkmani, a former minister of defense who headed the regime’s crisis management cell; and Shawkat, who was the deputy chief of staff of the Syrian military.

The three were attending a meeting of the central command unit for crisis management — a special cell composed of about a dozen of the country's top security chiefs established last year to oversee the crackdown on the uprising — at the National Security Building in the closely guarded Rawda neighborhood, home to many embassies, including the shuttered U.S. mission.

The official news agency SANA said that Interior Minister Mohammad Ibrahim al-Shaar and a man identified as Lt. Gen. Hisham, assumed to be the national security chief, Hisham Bakhtiar, were in stable condition after being injured.

The death of Shawkat, who was married to Assad’s elder sister Bushra, was an especially significant blow to the regime because of his standing as a member of the Assad family and as a leading figure in the effort to crush the uprising, Azm said.

“Assef Shawkat was not only a very close member of the Assad family but also a forceful and powerful member of the inner decision-making circle,” he said.

“He was well known for being brutal, effective and decisive, and he was at the forefront of the fight against the uprising.”

The circumstances of the attack remain unclear, however. The pro-government Al-Dunia TV station reported that it was a suicide attack. But commanders in the Free Syrian Army, which asserted responsibility, said a bomb had been planted inside the meeting room and was detonated remotely.

According to Col. Malik Kurdi, spokesman for the Free Syrian Army, the operation had been planned more than two months ago, and rebels seized the opportunity to act following the battles that erupted in the streets of Damascus over the past four days. “The circumstances were convenient and morale was high,” said Kurdi, speaking from a refugee camp in southern Turkey.

A separate claim of responsibility for the bombing came in a Facebook posting from a little-known group calling itself the Brigade of Islam.

Dehghanpisheh reported from Beirut. Suzan Haidamous and Ahmed Ramadan in Beirut and a special correspondent in Damascus contributed to this report.