Syrian government forces have bombarded the city of Homs with tank shells and mortars, killing scores of people, opposition groups say.
Activists say a massacre has occurred but the government denied attacking, accusing the opposition of propaganda.
A BBC correspondent outside Homs says opposition fighters, though outgunned, plan to launch a "general offensive".
US President Barack Obama sharply condemned the Homs attack, as diplomats met at the UN to discuss a resolution.
He accused President Bashar al-Assad of launching an "unspeakable assault" on his own citizens, and called for him to resign and for democratic elections to be held.
A vote is expected later on the UN draft resolution, despite Syrian ally Russia voicing strong objections.
Foreign Minister Sergei Lavrov said on Saturday the current draft included measures against President Assad's government, but not against armed opposition groups.
But Russian amendments to the text were described as "unacceptable" by the US ambassador to the UN, Susan Rice.
Mr Lavrov is due to have talks with Mr Assad in Damascus on Tuesday, along with the head of the Russian Foreign Intelligence Service, Mikhail Fradkov.
In another development, Tunisia moved to sever relations with the Assad government.
Early accounts of the casualties in Homs talked of as many as 200 deaths, but one of the main activist groups later revised its confirmed toll down to 55.
The Local Coordinating Committees said it had documented 39 deaths in the Khalidiya district of Homs, eight in other districts of the city and eight in the surrounding region.
Photos from the city on Saturday showed a mass funeral under way in Khalidiya.
Homs appears to have come under a "pretty relentless" bombardment, which targeted areas outside government control, the BBC's Paul Woods reports from just outside the city, where he is travelling with fighters from the Free Syria Army.
He says the fighters are trying to get in via back roads with blood supplies for the casualties.
"Yesterday the Syrian government murdered hundreds of Syrian citizens, including women and children, in Homs through shelling and other indiscriminate violence..." President Obama said in a statement in Washington.
"Assad must halt his campaign of killing and crimes against his own people now. He must step aside and allow a democratic transition to proceed immediately."
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 Mr 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.
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.
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
- Fresh protests began outside the Syrian embassy in London on Saturday afternoon after six arrests at a demonstration there overnight
- 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
- 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 Mr 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".