Russia and China have vetoed an Arab and western-backed resolution at the UN Security Council calling for Syrian president Bashar al-Assad to step down over his bloody crackdown on a popular uprising.
The setback in diplomatic efforts to defuse the revolt peacefully came after world leaders and Syrian opposition activists accused Dr Assad's forces of killing hundreds of people in a bombardment of the city of Homs, the bloodiest night in 11 months of upheaval in the pivotal Arab country.
Shortly before the Security Council voted, US president Barack Obama denounced the "unspeakable assault" on Homs and demanded that Dr Assad leave power immediately. He also called for UN action against Dr Assad's "relentless brutality".
"Yesterday the Syrian government murdered hundreds of Syrian citizens, including women and children, in Homs through shelling and other indiscriminate violence, and Syrian forces continue to prevent hundreds of injured civilians from seeking medical help," Mr Obama said in a statement. "Any government that brutalizes and massacres its people does not deserve to govern."
French foreign minister Alain Juppe called the killings in Homs a "massacre" and a "crime against humanity." In remarks clearly aimed at Moscow, he said any country that blocked UN action would bear a "heavy responsibility in history."
Death tolls cited by activists and opposition groups ranged from 237 to 260, making the Homs attack the deadliest so far in Dr Assad's crackdown on protests and one of the bloodiest episodes in the "Arab Spring" of revolts that have swept the region.
Residents said Syrian forces began shelling the Khalidiya neighbourhood at around 8 pm last night using artillery and mortars. They said at least 36 houses were completely destroyed with families inside.
"We were sitting inside our house when we started hearing the shelling. We felt shells were falling on our heads," said Waleed, a resident of Khalidiya.
"The morning has come and we have discovered more bodies, bodies are on the streets," he said. "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 neighbourhood said residents were using primitive tools to rescue people. They feared many more 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. Each one had a capacity to deal with 30 people, but he estimated the total number of wounded at 500.
"We have dug out at least 100 bodies so far, they are placed in the two mosques."
Syria denied shelling the area and said internet videos of corpses were staged. It is not possible to verify activist or state media reports as Syria restricts independent media access. The official Syrian account was disregarded across the globe, where condemnation was thunderous.
US secretary of state Hillary Clinton said today it had not been possible to work constructively with Russia ahead of the UN vote, even though military intervention in Syria - fiercely opposed by Moscow - had been absolutely ruled out.
"I thought that there might be some ways to bridge, even at this last moment, a few of the concerns that the Russians had. I offered to work in a constructive manner to do so. That has not been possible," she told reporters at the Munich Security Conference.
Moscow said before the vote that the resolution was not "hopeless," but its wording needed to be altered to avoid "taking sides in a civil war." Foreign minister Sergei Lavrov said it was still possible to reach consensus.
But US ambassador to the United Nations Susan Rice said amendments that Russia had proposed were "unacceptable."
After what US officials called "vigorous" talks between Ms Clinton and Mr Lavrov, Moscow announced that its foreign minister would fly to Syria in three days to meet Dr Assad.
As news of the violence spread, angry crowds of Syrians stormed their country's embassies in Cairo, London, Berlin and Kuwait and protested in other cities.
Tunisia announced it was expelling the Syrian ambassador and revoking recognition of Assad's government. The head of a committee of parliamentarians from Arab states said Arab countries should expel Syrian ambassadors and cut ties.
Turkish foreign minister Ahmet Davutoglu said: "If the Syrian administration is given the understanding that the current situation of hundreds of people dying daily can continue and the U.N. will not take a stance against it, the atmosphere of clashes will increase more."
It was not immediately clear what had prompted Syrian forces to launch such an intense bombardment, just as diplomats at the Security Council were discussing the draft resolution on Syria.
In Cairo, a crowd stormed the Syrian embassy, smashing furniture and setting fire to parts of the building in protest over the Homs bloodshed. The gate of the embassy was broken and furniture was smashed on the second floor of the building.
In London, 150 people hurled stones at the Syrian embassy, smashing windows and shouting slogans. Police said five men were arrested after breaking into the building and another held for assaulting police.
In the cities of Hama and Idlib, activists said hundreds of people took to the streets in solidarity. They chanted in Idlib: "Homs is bombarded, and you are still sleeping?"
Reuters