Russia and China have handed Syria's government a "licence to kill", say opposition activists, after the two countries vetoed a UN resolution.
The Syrian National Council, an opposition umbrella group, urged Moscow and Beijing to change their minds.
The veto caused anger among Western diplomats. Activists earlier attacked Syrian embassies across the world.
In Syria, dozens were reported killed on Saturday in one of the bloodiest days since protests began last March.
Rebel groups and opposition activists said the regime launched an assault on the city of Homs on Friday night using mortar and tanks to bombard civilian areas.
One opposition group said it had confirmed 62 deaths in Homs, while other organisations gave death tolls in excess of 200.
Activists said mass protests in solidarity with Homs residents had continued overnight in cities around the country.
Violence was also continuing, particularly in the north, where nine security personnel were reported killed in clashes in Idlib province.
In a statement, the Syrian National Council condemned Moscow and Beijing for obstructing the passage of the draft resolution.
"The SNC holds both governments accountable for the escalation of killings and genocide, and considers this irresponsible step a licence for the Syrian regime to kill without being held accountable," the statement said.
The resolution, which called for a "Syrian-led political transition to a democratic, plural political system", was backed by the 13 other members of the Security Council.
The veto drew an angry response from American UN envoy Susan Rice, who said that she was "disgusted" and said Russia and China would have blood on their hands.
UK Foreign Secretary William Hague said the two countries were making a "great mistake", accusing them of "turning their backs on the Arab world".
Tunisia, which had already indicated its intention to break diplomatic ties with Damascus, called on Sunday for all Arab states to follow their lead.
But Russia's Deputy Foreign Minister Gennady Gatilov defended Moscow's position.
"The authors of the draft Syria resolution, unfortunately, did not want to undertake an extra effort and come to a consensus," he wrote in a Twitter message.
China's state news agency Xinhua ran a commentary piece saying the two countries believed more time and patience was needed to solve the crisis.
The double veto "aimed at further seeking peaceful settlement of the chronic Syrian crisis and preventing possible drastic and risky solutions", said the commentary by Yu Zhixia.
Russia's Foreign Minister Sergei Lavrov is due to visit Damascus on Tuesday, to encourage democratic reform, Russian officials say.
Syria has been gripped by nationwide protests against President Bashar al-Assad's government for almost a year.
Opposition groups say more than 7,000 civilians have died in the struggle.
The government characterises the unrest as a conflict with armed gangs and terrorists, and says several thousand security personnel have been killed.
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