Widespread fighting raged in Damascus Thursday as Assad regime forces and militias exacted vengeance for a bombing that killed three top Syrian security chiefs.
The fighting included regime soldiers firing indiscriminately in embattled neighborhoods and shelling buildings from helicopters, activists said.
Scores of people were killed, the activists said.
Residents reported pro-regime militiamen known as shabiha swarmed into several Damascus neighborhoods, intensifying already escalated violence that began Wednesday afternoon after three of about a dozen of the country's top security chiefs were killed in a conference-room bombing attack.
A witness in a neighborhood in Damascus' historic walled Old City told The Washington Post Wednesday, when the vengeance started, militiamen broke down doorways and killed families with knives.
"Shabiha militias are killing people with knives," a separate opposition activist told the Post. "There are tens of bodies on the streets."
The first witness said he watched from his rooftop as men and children ran into the streets with guns and knives to try to defend the neighborhood against the militiamen.
"Armed people are walking in the streets. I can't tell who they are or what God they believe in," a third resident told the Post by Skype.
After the late-morning bombing, Syria's military issued a statement saying, "This terrorist act will only increase our insistence to purge this country from the criminal terrorist thugs and to protect the dignity of Syria and its sovereignty."
The Defense Ministry said in a statement read over state TV, "If the people think they can force Syria in a certain direction by killing these people, they are delusional."
Some rebel commanders said they feared the regime would use its arsenal of chemical weapons against the people.
The regime said Wednesday's bombing attack inside the headquarters of Syria's national security council, a compound near President Bashar Assad's fortified palace, was the work of a suicide bomber. The Free Syrian Army said it was a remotely detonated explosive.
The FSA told the British newspaper The Daily Telegraph two bombs -- one made of 25 pounds of TNT and the other a smaller Composition C-4 plastic explosive -- were planted in the conference room days before the meeting by an opposition mole working for Gen. Hisham Ikhtiyar, one of Assad's intelligence chiefs.
"One was hidden in a packet of chocolates and one in a big flower pot that was in the middle of the table of the conference room," FSA logistical coordinator Louay al-Mokdad told the newspaper.
He claimed the operation was conducted by FSA members in collaboration with drivers and bodyguards working for Assad's inner circle -- a scenario repeated by other activists.
The dead included Defense Minister Dawoud Rajha; former Defense Minister Hassan Turkmani, who headed the regime's crisis management cell; and Asef Shawkat, Assad's brother-in-law and deputy chief of staff of the Syrian military.
Shawkat was widely believed to be the most significant victim. He was the husband of Assad's older sister, Bushra, and was one of the most feared figures in Assad's inner circle.
Assad's whereabouts were unclear. An unconfirmed report said he had been wounded and was flown to principal port city of Latakia.
The regime said in a statement Assad appointed Gen. Fahd Jassem al-Freij, deputy commander in chief of the armed forces, as his new defense minister.
In Washington, Defense Secretary Leon Panetta said Syria was "rapidly spinning out of control" and warned the regime to safeguard its chemical weapons stockpile.
"It's obvious what is happening in Syria is a real escalation," he said at a joint news conference with visiting British Defense Secretary Philip Hammond.
The White House said President Barack Obama and Russian President Vladimir Putin discussed the deteriorating situation in Syria in a rare phone call Wednesday.
Obama cautioned Putin that maintaining Russia's alliance with the Assad regime would put his country on the "wrong side of history," spokesman Jay Carney said.
Russia's Foreign Ministry condemned the bombing, calling it a "heinous crime" and "another attempt to further destabilize the situation in Syria."