The veto and the show of support by Russia raised concerns that Assad's regime could now unleash even greater violence to crush the revolt against his rule, assured that his ally would prevent international action while continuing its weapons sales to Damascus.
It could also push an opposition despairing of other options further into an armed response, fueling a cycle of violence that threatens to tear apart the Arab nation. A movement that began with peaceful protests in March has already turned increasingly to the weapons of rebel soldiers to defend itself against Assad's crackdown.
The overnight onslaught on restive neighborhoods in Homs, Syria's third largest city, signaled a willingness by Assad's regime to bring a new level of violence to stamp out its opponents. Its timing, hours before a planned vote on the U.N. resolution, suggested Assad was confident of his ally Russia's protection on the world stage.
Activists' reports of the death toll from the assault could not be independently confirmed.
The Syrian government denied any bombardment took place at all, saying the reports were opposition propaganda aimed at pressuring the United Nations. It said bodies of the dead that appeared in activists' online videos were those of people who had been kidnapped previously by "terrorists."
Residents of Homs on Saturday described a night of relentless bombardment by mortars and rockets that lasted until dawn, sending them fleeing to lower floors and basements. When daylight came, dozens of buildings were left punctured by shells, facades collapsed, and some streets were stained with blood.
Thousands gathered for a funeral ceremony for some of the victims in the worst hit neighborhood, Khaldiyeh, where more than 60 coffins and bodies in white shrouds were lined up in a park, according to footage of the scene.
"A few more nights like this one and Homs will be erased from the map," Ammar, a resident, said, speaking on condition that only his first name be used for fear he and his family could be targeted. "We are being massacred."
The bloodshed added new urgency to negotiations over the resolution, as Western and Arab nations amended drafts to overcome Russia's opposition.
"The Assad regime must come to an end," President Barack Obama said in a statement Saturday before the vote, calling on the Security Council to "stand against the Assad regime's relentless brutality."
Early drafts demanded Assad carry out an Arab League peace plan by which he would hand over his powers to his vice president and allow formation of a unity government. That was amended to an expression of support of the plan without detailing its provisions. Also added were calls for "all armed groups" — a reference to army defectors — to stop violence.
But Russia demanded further changes, saying the draft did not make enough demands on the armed opposition in Syria and that calls for Assad to step aside could wreck chances for a negotiated solution to the country's upheaval.
In the end, the resolution's proponents pushed ahead with a vote, challenging Moscow to veto or back down.
After the double veto, U.S. Ambassador Susan Rice said her country was "disgusted" by the vote.
French Ambassador Gerard Araud said Russia and China had "made themselves complicit in a policy of repression carried out by the Assad regime."
Russian Ambassador Vitaly Churkin accused fellow council members of being inflexible, saying proposed Russian amendments had been ignored. Chinese Ambassador Li Baodong said his country joined Russia in voting against the resolution for the same reason.
Moroccan Ambassador Mohammed Loulichki, a key sponsor of the resolution, said he still hoped that consensus could be reached later on a Security Council measure to stop the violence.
Syria has been a key Russian ally since Soviet times, and Moscow remains a key arms supplier to Damascus. Russia has opposed any U.N. call that could be interpreted as advocating military intervention or regime change. Russia and China also used their veto powers in October to block an attempt to condemn the violence in Syria.
In a glimpse into his thinking, a pro-Syrian Lebanese politician who met with the Syrian leader last week told a Lebanese newspaper that Assad was "confident in the Russian position."
Wiam Wahhab said Assad told him that the time had come to decisively put an end to the uprising. "The price of chaos is worse than the price of decisiveness," he quoted Assad as saying.
George A. Lopez, a professor at the University of Notre Dame's Kroc Institute for International Peace Studies, said Assad will likely see the veto as a green light to step up a crackdown that has killed well over 5,400 people since March, according to a U.N. estimate.
"I expect the regime will now unleash all the guns — literally — as the global condemnation has not occurred," he said.
With the resolution's defeat — and the apparent death knell of the Arab League peace plan — Burhan Ghalioun, head of the Syrian National Council, the main opposition umbrella, called for "friendly nations," particularly Arabs, to form a coalition to "support the Syrian people in all ways that the liberation of Syria requires."
That could include, "if necessary, support for the Free Syrian Army to protect the Syrian people," he told Al-Arabiya TV.
The Free Syrian Army is a force made up of army defectors who initially mainly acted as a defense for protesters. But in recent months, it has stepped up offensive attacks on regime forces and has grown increasingly bold in attempts to overtly control opposition-dominated cities, towns and neighborhoods.
It is believed to number in the thousands of fighters, though its true size is unknown, and it is heavily outgunned by regime forces.
Last week, regime troops carried out a major offensive that crushed FSA fighters who took control of suburbs of Damascus, bringing them to the doorstep of the capital.
There were signs that the bombardment in Homs was in response to moves by rebels.
Residents reported that the fighters set up new checkpoints in several areas, and two Homs activists said defectors attacked a military checkpoint in the Khaldiyeh district Thursday night, capturing 17 soldiers. The activists spoke on condition of anonymity to protect themselves from retaliation.
On Saturday, thousands protested across Syria in solidarity with the beleaguered city. "Homs, your blood will not go in vain," read a banner held by a protester a Damascus suburb.
At least 21 people were killed in violence outside Homs on Saturday, including 12 shot when security forces opened fire on a funeral procession for victims of a shooting in the Damascus suburb of Daraya a day earlier, according to the British-based Syrian Observatory for Human Rights.
Tunisia decided to expel Syria's ambassador and end its recognition of Assad's regime in response to what it called a "bloody massacre" in Homs. Angry Syrians stormed their embassies in Berlin, London, Athens, Cairo and Kuwait, clashing with guards and police and — in Cairo — setting fire to part of the embassy. Syrian exiles and supporters in Libya occupied the already-vacated embassy in Tripoli, saying it was now theirs to use.
In Homs, residents of the overwhelmingly Sunni Muslim district of Khaldiyeh checked on relatives after a night spent in hiding and cleaned streets of shattered glass, debris and bloodstains.
Online video by activists during the onslaught showed chaotic scenes in a makeshift clinic in what appeared to be a Khaldiyeh mosque — a room filled with wounded men with gashes and broken limbs being bandaged, as well as several dead bodies. In another video, fire ravaged a house that had been shelled.
The videos could not be independently verified.
Residents said most shelling came from a military installation west of Khaldiyeh and Alawite-dominated neighborhoods to the east. Syria's Alawite minority, which belongs to an offshoot of Shiite Islam, forms the backbone of Assad's regime and the military leadership.
Homs has been one of the biggest centers of anti-regime protests and has been hit by near daily regime raids. It has also seen bloody bouts of tit-for-tat killings between its Alawite and Sunni communities, a harbinger of what many Syrians fear could happen if the country descends into an outright civil war.
The Syrian Observatory said the death toll in Homs was at least 217, counting victims whose names it had collected. About 140 of the deaths were in Khaldiyeh, it said. The Syrian National Council put the toll at more than 220.
Mohammad, a Khaldiyeh resident who like most in Homs declined to be further identified, said the shelling started shortly before midnight and lasted until early Saturday.
"We were sitting at home and the mortars just started slamming into buildings around us," he said by telephone. "It's a catastrophe, no other way to describe it."
Snow reported from the United Nations. Associated Press writers Aya Batrawy in Cairo and Bouazza ben Bouazza in Tunis contributed to this report.