More than 200 people were killed by Syrian government forces which bombarded the city of Homs with tank shells and mortars, opposition groups say.
Activists say a massacre has occurred but the government denied attacking, accusing the opposition of propaganda.
But opposition fighters, though outgunned, plan to launch a “general offensive”.
A vote is expected later on a UN draft resolution, despite Syrian ally Russia voicing strong objections.
Foreign Minister Sergei Lavrov said on Saturday the current draft included measures against President Bashar al-Assad’s government, but not against armed opposition groups.
Sergei Lavrov said Moscow had tabled amendments to the text to try and ensure the UN would not appear to be taking sides in a civil war.
Homs appears to have come under a “pretty relentless” bombardment, which targeted areas outside government control.
Funerals have already begun in large numbers, amid reports of a death toll as high as 260.
Russia’s foreign minister said it would be a “scandal” to ask the UN Security Council to vote on the resolution in its current form.
Russia is Syria’s main ally on the council, and has said it will veto any resolution calling on President Bashar al-Assad to stand down.
Moscow has continued to supply weapons to Syria despite the protests.
However, French Foreign Minister Alain Juppe said the Homs assault was a “further step in savagery” and warned against a veto.
“Those who would hinder the adoption of such a resolution would assume a heavy responsibility in history,” he said.
In another blow to the Assad regime, Tunisia’s government said it had begun the process of expelling the Syrian ambassador and withdrawing recognition from the government.
A death toll higher than 200 at Homs would make it by quite a long way the bloodiest day since protests began.
Homs was one of the first cities to join anti-Assad protests, and became one of the focal points of dissent after government forces fired on crowds in April last year. Many army defectors have sought refuge in the city.
Activists said most of the deaths were in the residential area of Khalidiya.
Reports said a hospital had been destroyed in Khalidiya, and residents said more than 30 houses had been wrecked in the barrage.
“We were sitting inside our house when we started hearing the shelling. We felt shells were falling on our heads,” Khalidiya resident Waleed told Reuters news agency.
Video footage emerged on the internet showing several bodies covered in blood with a voiceover saying the bombardment was still going on.
State media dismissed the Homs casualty reports as a “hysterical campaign of incitement” by armed gangs designed to influence the UN.
“The civilians shown by satellite television stations are citizens who were kidnapped and killed by armed gunmen,” said a report on Sana news agency.
International media outlets are restricted in Syria, making it difficult to verify the claims of either side.
Meanwhile, activists have attacked Syrian embassies around the world
• Around 50 mainly Syrian protesters broke into their country’s embassy in Athens early on Saturday, smashing windows and
• About 20 protesters forced their way into the Syrian embassy in Berlin late on Friday and damaged offices, police said. German TV showed a Syrian resistance flag hanging from one of the windows and graffiti sprayed on the outside of the building
• About 150 demonstrators gathered outside the Syrian embassy in London early on Saturday and five people were arrested after entering the building
• In Cairo, protesters stormed the embassy building, smashing furniture and setting fire to parts of the building
Syria has been gripped by nationwide protests against Bashar al-Assad’s regime for almost a year.
The UN stopped estimating the death toll after it passed 5,400 in January, saying it was too difficult to confirm numbers.
The Syrian government says at least 2,000 members of its security forces have been killed fighting “armed gangs and terrorists”.
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