Syrian forces killed nearly 140 people including at least 100 when the army stormed the flashpoint protest city of Hama, activists said, prompting calls for emergency Security Council talks.
Activists said it was one of the deadliest days in Syria since demonstrators first took to the streets on March 15 demanding democratic reforms before turning their wrath on the regime and calling for its ouster.
As reports of the brutal pre-Ramadan crackdown on Hama unfurled, US President Barack Obama and European leaders condemned the crackdown as Germany and Italy called for an urgent meeting of the UN Security Council.
A meeting could be held Monday. But the move is likely to reopen bitter divisions within the Security Council, which has not yet been able to agree even a statement on President Bashar al-Assad's crackdown against opponents.
"It is one of the deadliest days" since the protests erupted, said Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
Rights activists said at least 136 people were killed across Syria and expected the toll to rise, while scores were wounded.
"One hundred civilians were killed on Sunday in Hama by gunfire from security forces who accompanied the army as it stormed the city," said Abdel Karim Rihawi, head of the Syrian League for the Defence of Human Rights.
Rihawi said five other people were killed in the central city of Homs and three more in the northwestern province of Idlib when security forces opened fire on protesters who rallied in support of Hama.
The head of the National Organisation for Human Rights, Ammar Qorabi, put the Hama death toll at 95. The Observatory's Abdel Rahman said at least 47 people were killed in and around the central city.
"The number of those wounded is huge and hospitals cannot cope, particularly because we lack the adequate equipment," he added, quoting a Hama hospital source.
Abdel Rahman said the Hama crackdown came after more than 500,000 people rallied in the city on Friday after Muslim prayers during which a cleric told worshippers "the regime must go".
Activists also reported deaths in Deir Ezzor, Syria's main gas- and oil-production hub in the east, which has become a rallying point for protests along with Hama.
At least 19 people were killed in Deir Ezzor, six in Herak in the south, and one in Al-Bukamal in the east, said Qorabi, adding most of those shot in Deir Ezzor were "hit in the head and the neck" by snipers.
Abdel Rahman meanwhile told AFP that protesters set ablaze 24 army troop carriers in the Masrib region west of Deir Ezzor. "They threw Molotov cocktails on a military convoy to stop it from advancing on Deir Ezzor and set ablaze 24 troop carriers."
The Syrian Revolution 2011, an Internet group that has been a driving force behind the protests, urged demonstrators to gather nationwide after Ramadan "taraweeh" evening prayers later Sunday "for retaliation protests".
"Syria is bleeding" it said.
India will take over from Germany on Monday as president of the Security Council, fixing the body's agenda. But a German UN official said he expected a meeting on Monday afternoon.
Britain, France, Germany, Portugal and the United States have been pressing for weeks for some kind of condemnation of the violence.
But Russia, China, South Africa, India and Brazil -- which are angry at the NATO bombing campaign in Libya -- have refused to support the move.
Russia and China have threatened to veto any formal resolution against Assad.
European diplomats said that the latest violence could sway one of the doubters to back a Security Council move.
India, South Africa and Brazil have told other council members that they plan to send a joint mission to Syria to discuss the crisis, diplomats said. Deputy foreign ministers from each country would go on the mission.
UN leader Ban Ki-moon called again on Syria to halt the military offensive and strongly condemned the action by Assad, who has been refusing to take the UN leader's calls for several weeks, his spokesman said.
Western powers condemned the violence amid warnings of fresh sanctions against Assad's regime.
US President Barack Obama said he was "appalled by the Syrian government's use of violence and brutality against its own people" and paid tribute to the "courageous" demonstrators who have taken to the streets.
"In the days ahead, the United States will continue to increase our pressure on the Syrian regime, and work with others around the world to isolate the Assad government and stand with the Syrian people," Obama said.
A US diplomat in Damascus told BBC World Service radio that the violence in Hama amounted to "full-on warfare" and an act of desperation.
"There is one big armed gang in Syria, and it's named the Syrian government," said JJ Harder, the press attache at the American embassy in Damascus.
Italian Foreign Minister Franco Frattini called for an urgent meeting of the Security Council to "adopt a very firm position" and also suggested that EU ambassadors in Damascus meet on Monday in the Syrian capital.
Germany threatened to slap new sanctions on Damascus along with its EU partners as France warned that Syria's leaders "will have to answer for their deeds".
Syria's neighbour Turkey said it was "deeply saddened and disappointed... by the current developments on the eve of holy month of Ramadan".
Residents in Hama said the army entered the city with tanks at around 6:00 am (0300 GMT) before gunfire erupted, in an apparent operation to wrest back control after security forces withdrew almost two months ago.
The official SANA news agency charged that gunmen shot dead two security forces in Hama while a colonel and two soldiers were "martyred" in Deir Ezzor.
SANA said the gunmen torched police stations and attacked private and public property in Hama, adding soldiers tore down barricades and checkpoints set up by the armed men at the city's entrance.
Abdel Rahman said the army also launched an operation against Muadhamiya in the Damascus region at dawn, "with tanks blocking the southern, eastern and western entrances to the town".
In 1982, an estimated 20,000 people were killed in Hama when the army put down an Islamist revolt against the rule of Assad's late father, Hafez.
The president replaced the governor of Hama after 500,000 protesters rallied in the opposition bastion on July 1 calling for the fall of the regime.
At least 1,583 civilians and 369 members of the army and security forces have been killed since mid-March in Syria, according to the Observatory.