At least 88 people, including many children, have been killed in Syria's restive Homs province, opposition activists say, calling it a "massacre".
They said scores were wounded in the violence in Houla, as government forces shelled and attacked the town.
If the toll is right, it would be one of the bloodiest attacks in one area since a nominal truce began in April.
Meanwhile, UN chief Ban Ki-moon said the opposition controlled "significant parts of some cities".
In a letter to the Security Council, Mr Ban said the situation remained "extremely serious" and urged states not to arm either side in the conflict.
At least 20 others were killed in violence elsewhere in Syria on Friday, according to activists, after tens of thousands of protesters took to the streets after prayers.
In Houla, activists said some of those who died were butchered by government militia, others killed in shelling or summary executions.
The Local Coordination Committees (LCC) said at least 88 people had died, "most of them women and children".
The Syrian National Council urged the UN Security Council to act urgently after it said more than 110 people died, AFP news agency reported.
Activists have called a day of mourning.
The BBC's Jim Muir in Beirut says unverifiable video posted on the internet by activists showed the bloodstained and mangled bodies of many children huddled on a floor in the dark, with the commentator shouting that there were too many to count.
Activists said several whole families were slaughtered by security forces on the edge of town. Some died in heavy shelling, while others who were caught were summarily executed, our correspondent says.
In one instance, six members of a family were killed when their house was shelled, the Observatory said.
International media cannot report freely in Syria and it is impossible to verify reports of violence.
Earlier, a spokesman for UN-Arab League peace envoy Kofi Annan said he was planning to visit Damascus.
The spokesman declined to give a date, but diplomats in Geneva told AFP that the former UN secretary general would make the trip early next week.
'Evidence of shelling'
In his letter, Mr Ban also said that, judging from the sophistication of the attacks, "established terrorist groups" could have been behind some of the recent bomb blasts in Syria.
Earlier this month, a bombing in Damascus left 55 dead in an attack which the government blamed on al-Qaeda. The attack came amid mounting fears that the terrorist group was taking advantage of the conflict to gain a foothold.
Mr Ban said UN efforts to end the conflict had seen only "small progress", adding that the "overall situation in Syria remains extremely serious".
Mr Ban said Syria "has not ceased the use of, or pulled back, their heavy weapons in many areas" - one of the requirements of Mr Annan's peace plan.
"On several occasions, UNSMIS has heard the sound, or seen evidence, of shelling in population centres," he said.
On Thursday, a UN-mandated panel said Syrian security forces were to blame for most abuses in the conflict, which has continued despite the presence of UN observers, who now total 260.
Mr Annan's six-point peace agreement ordered a cessation of violence on 12 April. While casualties appeared to fall after the truce, the fighting quickly resumed to previous levels.
The UN says at least 10,000 have been killed since an uprising began in March 2011 against President Bashar al-Assad's rule.