Russia and China have vetoed a United Nations resolution calling for Syrian President Bashar al-Assad to stand down.
The two permanent members of the Security Council rejected the resolution despite reports of nearly 300 deaths in Homs following a night of shelling by the Syrian army and security forces.
Following the vote, the French ambassador to the UN, Gerard Araud, said: "It is a sad day for the council. It is a sad day for Syria... History has compounded our shame."
Susan Rice, the US ambassador to the UN, called the veto "shameful", adding that Russia and China had sold out the Syrian people in order to "shield a craven tyrant".
The draft resolution called for a "Syrian-led political transition to a democratic, plural political system". However, Russia and China said the draft was "unbalanced".
In reaction to the vote, Britain accused Russia and China of encouraging the Syrian regime's "killing regime".
Foreign Secretary William Hague joined international condemnation of their stance. He said: "Russia and China faced a simple choice today: would they support the people of Syria and the Arab League or not?
"They decided not to, and instead sided with the Syrian regime and its brutal suppression of the Syrian people in support of their own national interests.
"Their approach lets the Syrian people down, and will only encourage President Assad's brutal regime to increase the killing, as it has done in Homs over the past 24 hours."
Hague said the assault on Homs - which has been at the centre of the pro-democracy protests - was "all the more chilling" as it comes on the 30th anniversary of the Hama massacre.
Around 20,000 people died in the 1982 operation by the Syrian army - then under the orders of president Hafez Assad - the father of the present leader.
The UN says around 6,000 people have died so far in the crackdown on protests which began last year.
"More than 2,000 people have died since Russia and China vetoed the last draft resolution in October 2011," said Hague following the vote.
"How many more need to die before Russia and China allow the UN Security Council to act?
"Those opposing UN Security Council action will have to account to the Syrian people for their actions which do nothing to help bring an end to the violence that is ravaging the country.
"The United Kingdom will continue to support the people of Syria and the Arab League to find an end to the violence and allow a Syrian-led political transition."
Russia's ambassador vetos the draft resolution
Russia and China were condemned in a series of outspoken attacks by the other members of the Security Council - with the UK's outrage expressed by its ambassador, Mark Lyall Grant.
The violence had "ferociously escalated" in the last 24 hours, he said, accusing the two nations of failing in their duty.
"Those that blocked Council action today must ask themselves how many more deaths they are prepared to tolerate before they support even modest and measured action," he said.
"We removed every possible excuse. The reality is that Russia and China have today taken a choice: to turn their backs on the Arab world and to support tyranny rather than the legitimate aspirations of the Syrian people.
"They have failed in their responsibility as permanent members of the Security Council and they have done so on the most shameful day of the Syrian killing machine's 300 days of repression."
The draft resolution, tabled by Morocco, did not impose sanctions or authorise military action and contained nothing that warranted opposition, he said.
The Tunisia president announced that it would withdraw its recognition of the Assad's leadership and expel the Syrian ambassador as the anger mounted.
Earlier on Saturday, President Obama called the killing in Homs an "unspeakable assault", while Hague accused the Syrian regime of "cold-blooded cynicism".
Before the vote, the UK Mission to the UN said Security Council members were gathered in the chamber at the organisation's New York headquarters.
"We cannot afford further delay on action on the tragedy," a spokesman said on Twitter.
The ongoing resistance from Russia was expected, with Moscow a long-time ally of Damascus,
Violent protests outside the embassy in Belgrave Square, central London, were mirrored in several countries across Europe and the Middle East - including Libya and Tunisia which have already overthrown regimes in the so-called Arab Spring.
Human rights campaigner Peter Tatchell said: "The UK should expel the Syrian ambassador and press the International Criminal Court to draw up a warrant for the arrest of President Assad on charges of crimes against humanity.
"It is intolerable that we allow the Syrian ambassador to remain in London after this latest massacre of civilians by his government."
Chinese Ambassador to the United Nations Li Baodong sits with his hands down