Syrian state TV is reporting the deaths of at least 120 security personnel in battles with hundreds of gunmen in the north-western town of Jisr al-Shughour.
More than 80 of the deaths were said to have happened when the security headquarters in the town was overrun.
Communications are largely cut off and there has been little information from the protesters' side about the unrest.
If confirmed, it would be the deadliest day for the security forces since anti-government protests began in mid-March.
The reports came a day after human rights activists said at least 35 residents and police had been killed in Jisr al-Shughour.
The government launched a crackdown on Saturday, following demonstrations against President Bashar al-Assad after Friday prayers.
On Friday evening, Information Minister Adnan Mahmoud admitted the security forces had "intermittently" lost control of areas around Jisr al-Shughour, which lies about 20km (12 miles) from the Turkish border.
Earlier, in a dramatic series of urgent flashes, state TV said hundreds of gunmen who took over the town had committed "a real massacre".
A report said the fighting had begun at dawn, when an armed gang ambushed police as they approached the town "to rescue citizens being terrorised". Twenty officers reportedly died.
It also said another 82 personnel were killed when the town's security headquarters was overwhelmed, and eight in a bomb attack on a post office they were guarding.
Many government buildings were attacked and burned, causing further casualties, it added. The overall death toll for security forces was put at 120.
State TV said the gunmen were armed with medium-calibre weapons and grenades, and that they had stolen five tons of explosives from a nearby dam.
The official Sana news agency quoted a correspondent as saying reinforcements had been sent to Jisr al-Shughour and that security forces personnel were surrounding houses from which armed men were firing weapons.
He added that the gunmen were using civilians as human shields, and that some of their victims' bodies had been mutilated or thrown in the Orontes river.
"We will deal strongly and decisively, and according to the law, and we will not be silent about any armed attack that targets the security of the state and its citizens," said Interior Minister Ibrahim Shaar.
A rebellion in Jisr al-Shughour in 1980 against President Hafez al-Assad, Bashar al-Assad's father, was brutally crushed with scores of deaths.
Foreign media are greatly restricted in Syria and the reports have not been independently verified.
But opposition websites insisted that their protests had been peaceful, and they scorned the government's talk of armed gangs.
A Jisr al-Shughour resident told BBC Arabic that the protesters did not have any weapons and blamed the authorities for Monday's deaths.
"The soldiers were coming our way, then they were shot in the back by some Syrian security elements," Abu Nadir said.
Another told the Reuters news agency that people had set fire to the post office after snipers shot at those attending a mass funeral on Saturday.
Others said that members of the armed forces had changed sides and joined the protests. Some also said the town had been relatively calm until a mutiny at the security headquarters, where so many deaths were reported.
"I think they executed policemen who refused to open fire on demonstrators. There was a mutiny in the security service," one told the AFP news agency.
The BBC's Jim Muir in Beirut says if official reports are correct, it would be the first time officials have admitted to such a large loss of personnel.
The unrest began in the southern city of Deraa before spreading.
Activists say more than 1,100 people have been killed in the unrest.
Mr Assad, whose family has been in power for four decades, has promised reforms, but this has done little to placate his opponents.