Western and Arab countries responded with outrage after Russia and China vetoed a UN Security Council resolution that would have urged Syrian President Bashar al-Assad to give up power.
US Secretary of State Hillary Clinton called the veto a "travesty".
The vote came a day after activists say Syrian forces bombarded the city of Homs, killing more than 200 people in the worst night of bloodshed of the 11-month uprising.
Russia said the resolution was biased and would have meant taking sides in a civil war. Syria is Moscow's rare ally in the Middle East, home to a Russian naval base and a customer for its arms. China's veto was widely seen as following Russia's lead.
Washington's UN ambassador Susan Rice said she was "disgusted" by Russia and China's vetoes, and "any further bloodshed that flows will be on their hands".
Britain's Foreign Minister William Hague said Moscow and Beijing had turned their backs on the Arab world.
France's Alain Juppe said they "carried a terrible responsibility in the eyes of the world and Syrian people."
All 13 other members of the Security Council voted to back the resolution, which would have "fully supported" an Arab League plan under which Mr Assad should cede powers to a deputy, withdraw troops from towns and begin a transition to democracy.
The Western criticism was echoed in the Middle East, where Arab powers like Saudi Arabia and non-Arab Turkey have turned decisively against Mr Assad in recent months.
Arab League intends to build support for plan
Arab League head Nabil Elaraby said the body still intends to build support for its plan. The veto "does not negate that there is clear international support for the resolutions of the Arab League," he said in a statement.
The Security Council's sole Arab member, Morocco, voiced "great regret and disappointment" at the veto. Ambassador Mohammed Loulichki and said the Arabs had no intention of abandoning their plan.
Syria says it is being targeted by the West and by hostile neighbours providing diplomatic cover for an armed insurgency steered from abroad.
If activists' accounts are accurate, the bombardment of Homs on Friday night was one of the bloodiest episodes of the Arab Spring uprisings sweeping the region and the deadliest incident in the Syrian conflict.
Syrian activist groups gave varying tolls above 200 killed, saying tanks and artillery blasted the Khalidiya neighbourhood.
Damascus denies firing on houses and says images of dead bodies on the Internet were staged. Western governments say they believe the activists.
"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," President Barack Obama said in a statement before the UN Security Council vote.
"Any government that brutalises and massacres its people does not deserve to govern," Mr Obama said.
Syrian newspaper hails UN resolution veto
An official Syrian newspaper has hailed Russia and China's veto of the UN draft resolution.
The daily government mouthpiece Tishrin called the veto "a catalyst that will enable Syria to accelerate reforms, organise a referendum on a new constitution, multi-party elections and the formation of a larger government that includes opposition movements."
It said the veto would also "encourage certain countries to review their positions on the Syrian crisis," and demonstrate once again that "only Syria can resolve this crisis politically, shielded from all outside interference."
The official SANA news agency said "scores of Syrians" gathered outside the Russian and Chinese embassies in Damascus after the resolution was blocked "in a show of appreciation of Russia and China's stances at the Security Council."
At least 6,000 people are reported to have been killed since the opposition to the regime erupted in mid-March last year.
