One hundred and twenty policemen were killed by "armed gangs" in northwest Syria, state television said, as the authorities warned of a firm response.
Activists who spoke to AFP in Cyprus disputed the official account, speaking instead of a mutiny in the town of Jisrash Shugur, where security forces had been carrying out operations for three days.
"The armed groups are committing a real massacre. They have mutilated bodies and thrown others into the Assi river," the state broadcaster said. "They have burned government buildings."
It said a total of 120 police were killed, including 80 at the town's security headquarters, without specifying the date of the incidents in Jisrash Shugur.
Interior Minister Mohammed Ibrahim al-Shaar warned in a statement read on television that the authorities would hit back.
"The state will act firmly, with force and in line with the law. It will not stay arms folded in the face of armed attacks on the security of the homeland," he said.
In its first report on Monday of police deaths in the town, state television spoke of 20 officers killed in an ambush.
"The police and security agents are confronting hundreds of armed men. They have managed to liberate one district controlled by gunmen" in Jisrash Shugur, the channel said.
It said residents of the town, 330 kilometres (200 miles) north of Damascus, had "pleaded for help and the rapid intervention of the army."
"Armed gangs ambushed police who were on their way to rescue citizens being terrorised" by these gangs, state television said, adding that the groups were armed with "light weapons, grenades and are using residents as human shields."
Elsewhere, "eight guards at a post office were also killed by armed gangs, who used the building's gas pipes to blow it up," it added.
However, two activists who spoke to AFP in Nicosia said the town was calm on Monday, and spoke of a mutiny at a local security headquarters, where shooting was heard the day before.
"I think they executed policemen who refused to open fire on demonstrators. There was a mutiny in the security service," one said.
The other told AFP that "shooting followed by an explosion was heard in the military HQ, apparently after a mutiny."
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He said regime "snipers" had opened fire on protesters in the town, killing two. Demonstrators then gathered outside the headquarters and "shots and an explosion took place inside" the building, he added.
France and other western powers are ready to risk a veto by Russia at the United Nations over a draft resolution to condemn political violence in Syria, France's top diplomat said.
French Foreign Minister Alain Juppe, on a visit to Washington, said of the 15-member UN Security Council: "We think that it will be possible to get eleven votes in favor of the resolution and we'll see what the Russians will do."
The draft was drawn up by France, Britain, Germany and Portugal. It condemns violence at the hands of the regime of Syrian President Bashar al-Assad and asks him to open Syrian cities to humanitarian teams.
The five permanent council member countries -- Russia, Britain, France, the United States and China -- have veto power.
Juppe, speaking in English at the Brookings Institute, told diplomats and analysts that "if they veto, they will take their responsibility."
"Maybe if they see that there are eleven votes in favor of the resolution, they will change their mind. So there is a risk to take, and we're ready to take it," Juppe stressed, noting that he had discussed the situation with US Secretary of State Hillary Clinton.
Six human rights groups within Syria issued a joint statement condemning "the excessive use of force to disperse peaceful gatherings of unarmed Syrian citizens."
They urged the government to "stop the spiral of violence and assassinations in the streets of Syria," and demanded an independent and transparent inquiry to unmask those responsible.
"As the death toll in Syria reaches staggering new heights, it is imperative that the UN Security Council -- which has so far been silent on this issue -- votes to condemn the killings," said Philip Luther of Amnesty International.
"It must also take decisive action and refer the situation in Syria to the International Criminal Court," he said.
The latest violence comes despite Syria freeing more than 450 political prisoners since last Tuesday under a general amnesty announced by Assad.
Late on Sunday, the state news agency SANA also reported the creation of a committee tasked with drafting a law on political parties.
The current constitution stipulates that the ruling Baath party is "the leader of state and society," but political pluralism has been at the forefront of demands by pro-reform dissidents.
Rights groups say more than 1,100 civilians have been killed and at least 10,000 arrested in Syria since anti-regime protests erupted in mid-March.
Damascus blames the unrest on "armed terrorist gangs" backed by Islamists and foreign agitators.