Rebels seized control of sections of Syria's international borders and torched the main police headquarters in the heart of old Damascus, advancing relentlessly after the assassination of President Bashar al-Assad's closest lieutenants.
The battle for parts of the capital raged into the early hours of Friday. In some neighbourhoods, residents said there were signs the government's presence was diminishing.
Officials in neighbouring Iraq confirmed that Syrian rebels were now in control of the Syrian side of the main Abu Kamal border checkpoint on the Euphrates River highway, one of the major trade routes across the Middle East.
Rebels also claimed control of at least two border crossings into Turkey at Bab al-Hawa and Jarablus, in what appeared to have been a coordinated campaign to seize Syria's frontiers.
In Damascus, a witness in the central old quarter district of Qanawat said the huge headquarters of the Damascus Province Police was black with smoke and abandoned after being torched and looted in a rebel attack.
"Three patrol cars came to the site and were hit by roadside bombs," said activist Abu Rateb by telephone.
The next few days will be critical in determining whether Assad's government can recover from the devastating blow of Wednesday's bombing, which wiped out much of Assad's command structure and destroyed his circle's aura of invulnerability.
Assad's powerful brother-in-law, his defence minister and a top general were killed in Wednesday's attack. The head of intelligence and the interior minister were wounded.
Government forces have responded by blasting at rebels in their own capital with helicopter gunships and artillery stationed in the mountains overlooking it.
Assad's failure to appear in public for more than 24 hours - he was finally shown on television on Thursday swearing in a replacement for his slain defence minister - added to the sense of his power evaporating. His whereabouts are not clear.
Diplomatic efforts - rapidly overtaken by events on the ground - collapsed in disarray on Thursday when Russia and China vetoed a U.N. Security Council resolution that would have imposed sanctions unless Syrian authorities halted violence. Washington said the council had "failed utterly."
Activists in Damascus said rebels were now in control of the capital's northern Barzeh district, where troops and armoured vehicles had pulled out.
The army had also pulled out of the towns of Tel and Dumair north of Damascus after taking heavy losses, they said. But they said troops were hitting the western district of Mezzeh with heavy machineguns and anti-aircraft guns overnight.
The reports could not be confirmed. The Syrian government restricts access by international journalists.
A resident who toured much of Damascus late on Thursday said he saw signs the government's presence was diminishing, with only sporadic checkpoints and tanks in place in some areas. The Interior Ministry at the main Marjeh Square had a fraction of its usual contingent of guards still in place.
Shelling could be heard on the southwestern suburb of Mouadamiyeh from hills overlooking the city where the Fourth Division, commanded by Assad's brother Maher, is based, he said.
Syrian television showed the bodies of about 20 men in T-shirts and jeans with weapons lying at their sides, sprawled across a road in the capital's Qaboun district. It described them as terrorists killed in battle.
The operations to seize the border checkpoints appear to show a level of coordination and effectiveness hitherto unseen from the rebels, who have been outgunned and outnumbered by the army throughout the 16-month conflict.
Footage filmed by rebels at the Bab al-Hawa border crossing with Turkey showed them climbing onto rooftops and tearing up a poster of Assad.
"The crossing is under our control. They withdrew their armoured vehicles," said a rebel fighter who would only be identified as Ali, being treated for wounds on the Turkish side.
Two officers in the rebel Free Syrian Army said fighters were keeping themselves busy into the early hours of Friday, dismantling border computer systems, seizing security records and emptying the shelves of the duty-free shop.
At least 30 government tanks in the area had not mobilised to try to recapture the border post, according to Ahmad Zaidan, a senior Free Syrian Army commander.
Officials in neighbouring Lebanon said refugees were pouring across the frontier: a security source said 20,000 Syrians had crossed on Thursday.
Diplomacy has been largely ineffective throughout the crisis, with Western countries condemning Assad but showing no stomach for the sort of robust intervention that saw NATO bombers help blast Libya's Muammar Gaddafi from power last year.
Thursday's failed U.N. Security Council resolution, which would have extended a small, unarmed U.N. monitoring mission, was the third that has been vetoed by Russia and China.
With the mandate for the mission set to expire at 0400 GMT on Saturday, Western states that pushed the resolution to renew the operation under a threat of sanctions against Damascus reacted angrily to the vetoes.
The U.S. ambassador to the United Nations, Susan Rice, said the Security Council had "failed utterly", and Washington would look outside the body for ways "to bring pressure to bear on the Assad regime and to deliver assistance to those in need."
The Security Council was set for another showdown on Friday over new rival resolutions intended to simply extend the mission. Pakistan, with the support of Russia, is proposing a 45-day extension, while Britain has put forward a 30-day extension.
After negotiations late on Thursday, it was unclear if either measure had enough backing to pass - at least nine votes and no vetoes by the United States, Britain, France, Russia or China. The Security Council is scheduled to vote on Friday.
(Additional reporting by Oliver Holmes, Samia Nakhoul and Dominic Evans in Beirut, Suleiman Al-Khalidi in Cilvegozu, Turkey, and Michelle Nichols at the United Nations; Writing by Peter Graff; Editing by Andrew Roche and Peter Cooney)