Scores, possibly hundreds, of people are estimated to have been killed in Syria when President Assad's forces shelled homes in the opposition stronghold of Homs. The attack came on the eve of yesterday's United Nations Security Council vote on a resolution calling on Mr Assad to step down which was foiled, amid much recrimination, by a double veto from Russia and China.

The vetoes triggered blistering reprimands of both countries from the other 13 members in the chamber, all of whom had voted in favour. “The United States is disgusted,” the US envoy, Susan Rice, said in accusatory tones rarely seen at the UN. She added that Russia and China should consider themselves “complicit” in massacres committed by the Assad regime. “Any further blood that flows will be on their hands.

Britain's ambassador, Sir Mark Lyall Grant, said the UK was “appalled” by the failure of the resolution that also had the backing of the Arab League. “Those that blocked council action today must ask themselves how many more deaths they are prepared to tolerate. Russia and China have taken a choice to turn their backs on the Arab world and to support tyranny.”

There was confusion in Syria about the numbers killed. Activists said, in claims widely reported throughout most of yesterday, that hundreds had died. This would have made the Homs attack the deadliest since Mr Assad's crackdown began. But one group later revised their estimates down to “scores” of dead. Whatever the figure is, it will have to be added to a UN calculation some weeks ago that 5,400 people have died since last March. Opposition claims cannot be independently verified, but reports of the bombardment were endorsed by world leaders and immediately sparked worldwide reaction with attacks on, or demonstrations outside, Syrian embassies in London, Cairo, Berlin, Greece, the US and Kuwait. Tunisia said it was expelling Syria's ambassador in response to what it called “the bloody massacre” in Homs.

If London and Washington were calculating that the overnight news from Syria would make any veto impossible, they were wrong. China and Russia, which blocked a similar text in October, share an aversion to any resolution that promotes outside interference in internal strife, particularly if it involves regime change. Russia, which has strategic interests and a naval base in Syria and also supplies arms to the Assad regime, said earlier that passage of the UN draft would have been a “scandal”. It had asked for a series of amendments, for instance excising provisions inferring human rights abuses by the regime and others demanding it withdraw its heavy armaments from its own cities. Its foreign minister and an intelligence chief will go to the Syrian capital, Damascus, this week.

The timing of the attack on Syria's third city of 1.5 million people, the night before the UN vote, is striking. Residents say that Syrian forces began shelling the Khalidiya district of Homs at about 8pm on Friday using artillery and mortars. They said at least 36 houses were completely destroyed with families inside. Waleed, a resident of Khalidiya, said: “We were sitting inside our house when we started hearing the shelling. We felt shells were falling on our heads. The morning has come and we have discovered more bodies; bodies are on the streets. Some are still under the rubble. Our movement is better but there is little we can do without ambulances and other things.”

An activist in the district said residents feared many were buried under rubble. “We are not getting any help. There are no ambulances or anything. We are removing the people with our own hands,” he said, adding there were only two field hospitals treating the wounded. “Homs is on fire,” said an opposition activist in a quieter area of the city, who did not want to be identified for fear of reprisal. “All sides are attacking each other, and the number of casualties is more than anyone can count,” he said.

Ammar, a resident of the Bab Tadmur district of Homs, said: “A few more nights like this one and Homs will be erased from the map. We are being massacred. What is the Security Council waiting for?”

Syria denied shelling the area and said video of corpses was staged for propaganda prior to the UN vote. Footage posted online showed at least eight bodies assembled in a room, one of them with the top half of its head blown off. One video showed a chaotic scene as men, with various wounds and gashes, were being tended or were praying in what appeared to be a makeshift clinic. Another showed fire consuming a house as people desperately tried to put out the blaze with water.

Opposition groups were adamant that serious numbers of people had been killed by the government onslaught, although they were substantially fewer than their first claims. The local co-ordination committees said that by nightfall on Saturday they had documented 55 deaths.

It was not immediately clear what had prompted Syrian forces to launch such an intense bombardment. Some Syrian activists said the violence was triggered by a wave of army defections in Homs. Other similarly unconfirmed reports said gunmen, possibly army defectors, had attacked a military checkpoint in Khalidiya and captured 17 of its members, prompting intense clashes with the military.

Homs, Syria's third largest city, is a hotbed of dissent against Mr Assad's regime and is known to shelter a large number of army defectors known as the Free Syrian Army. The city has seen several crackdowns by security forces, but many parts of it remain outside government control. Mr Assad is trying to crush the revolt with a sweeping crackdown that has so far claimed thousands of lives, but neither the government nor the protesters are backing down. Clashes between the military and an increasingly bold and armed opposition have meant many parts of the country have experienced relentless violence. Mr Assad's regime has been intensifying an assault against army defectors and protesters.

For all the undoubted repressive nature of the Assad regime, there is a wider regional and religious context at work. Syria – three-quarters Sunni yet ruled by an Alawite Shia sect – is an ally of Iran. In addition to the sectarian dimension, there is a wider, strategic one, especially at present, with Iran's disputed nuclear ambitions causing so much tension with Israel, the US and Europe. On Friday, Israel's purported willingness to launch strikes on Iran's nuclear capability – readily promoted by US officials – was widely reported. Yesterday, in a move that had been signalled for some time, Iran's Revolutionary Guard began military exercises in the country's south, the latest show of force after threats to close the strategic Strait of Hormuz in retaliation for tougher Western sanctions.

In a searing statement issued before the UN vote, President Barack Obama said Mr Assad had displayed “disdain for human life and dignity” following the weekend attacks in Homs. “The Syrian regime's policy of maintaining power by terrorising its people only indicates its inherent weakness and inevitable collapse,” he said. “Assad has no right to lead Syria, and has lost all legitimacy with his people and the international community.”

World opinion: 'Russia and China are complicit in these massacres'

"The Syrian regime's policy of maintaining power by terrorising its people only indicates its inherent weakness and inevitable collapse. Assad has no right to lead Syria, and has lost all legitimacy."- Barack Obama, US President

"The United States is disgusted. Russia and China should consider themselves complicit in massacres committed by the Assad regime. Any further blood that flows will be on their hands." - Susan Rice, US ambassador to the UN

"Those that blocked the action must ask themselves how many more deaths they are prepared to tolerate. Russia and China have taken a choice to turn their backs on the Arab world and support tyranny."- Sir Mark Lyall Grant, UK permanent representative to the UN

"The Syrian authorities have jumped a new hurdle in savagery: the massacre in Homs is a crime against humanity and those responsible will have to answer for it." - Alain Juppe, French Foreign Minister

"As a tyrant in Damascus brutalises his own people, America and Europe stand shoulder to shoulder. We are united, alongside the Arab League, in demanding an end to the bloodshed and a democratic future for Syria." - Hillary Clinton, US Secretary of State