Syria's opposition called for a "day of mourning and anger" on Tuesday after almost 100 people, most of them civilians, reportedly died in spiralling violence ahead of a UN Security Council showdown.
US Secretary of State Hillary Clinton was leading a Western charge pressing Russia to back UN Security Council action to stop a crackdown on dissent the United Nations says has killed more than 5,400 people in the past 10 months.
But Russia, which has veto power in the council, has objected to a resolution introduced by Morocco under which Syrian President Bashar al-Assad would accept a ceasefire and hand over power to a deputy ahead of talks.
In a statement on its Facebook page, the opposition Syrian National Council deplored the international community's lack of "swift action" to protect civilians "by all necessary means."
On Monday, "the regime waged a bloody campaign of massacres and terrorism that killed 100 Syrians including women and children... in Homs, using tanks and heavy weapons to bomb neighbourhoods," it said, referring to the central city.
It called, in coordination with activists, for a "day of mourning and anger in the country to commemorate the victims of savage massacres," urging mosques and churches to support the cause with prayer calls and ringing bells.
The SNC, the most representative group opposed to Assad, reaffirmed the "people's determination to fight for their freedom and dignity," stressing they "will not give up their revolution, whatever the sacrifices."
"The regime is taking advantage of the cover provided to it by some regional and international parties to escalate its crackdown," it added, in a likely reference to Iran and Russia.
An upsurge in violence on Monday, mostly in the flashpoint of Homs, killed almost 100 people, including 55 civilians, said the Syrian Observatory for Human Rights.
The unrest, which also saw 25 soldiers killed, marked one of the bloodiest days of a revolt that erupted in March inspired by a wave of Arab uprisings that last year overthrew authoritarian leaders in Egypt, Libya and Tunisia.
Amid the escalating violence which prompted the Arab League to suspend its observer mission to Syria, Clinton, the head of the League and the British and French foreign ministers headed to New York to push forward a UN resolution.
"The United States condemns in the strongest possible terms the escalation of the Syrian regime's violent and brutal attacks on its own people," Clinton said in a statement announcing her trip to the UN.
"The Security Council must act and make clear to the Syrian regime that the world community views its actions as a threat to peace and security. The violence must end, so that a new period of democratic transition can begin."
European Union leaders at a Brussels summit unanimously voiced outrage over the bloodshed in Syria. EU President Herman Van Rompuy called on the Security Council to "take long overdue steps to bring an end to the repression."
British Prime Minister David Cameron, citing reports that more than 400 children have been killed in the crackdown, said: "It's frankly an appalling situation."
"It's time for all the members of the UN Security Council to live up to their responsibilities instead of shielding those with blood on their hands," Cameron said.
Syria's foreign ministry fired back, saying "the aggressive American and Western statements against Syria are escalating in a scandalous manner," and again blaming the recent violence on "armed terrorist groups."
Russia and China -- which have accused Western nations of misusing a UN mandate in their intervention to bring down Libyan dictator Moamer Kadhafi -- in October vetoed an earlier Western-backed draft resolution on Syria.
Russian Deputy Foreign Minister Gennady Gatilov expressed similar concerns about the latest draft resolution. Russia has longstanding ties to Syria and is the main supplier of weapons to Assad's regime.
"The draft has statements in it calling on the member states to stop arms deliveries to Syria," Gatilov told Interfax news agency.
"But there is no clear line between arms contraband that some countries engage in to support extremist forces in Syria, and the legal military-technical ties with this country," he said.
Russia has instead called for Assad's regime and the opposition to hold "informal contacts" in Moscow without any preconditions.
Asked about Russia's call for talks, White House spokesman Jay Carney said the United States supported a political solution but was "intensely discussing" with Russia the "real deterioration on the ground" in Syria.
"The regime has lost control of the country and will eventually fall," Carney said.
Ahead of the UN meeting, a French diplomatic source said "the balance within the Security Council has evolved" and that "at least 10" of the body's 15 members could vote in favour of the new draft resolution.