Syrian troops backed by tanks and heavy armour have stormed the southern town of Deraa and also Douma, a suburb of the capital Damascus, resulting in many deaths and dozens of arrests.
Security forces loyal to Bashar al-Assad, the country's president, have also continued a crackdown in the coastal town of Jableh for a second day.
An activist said late on Monday that 18 people had been killed in Deraa alone.
However, the government insists the army was invited in to rid the town of gunmen.
Al Jazeera's Rula Amin, reporting from Damascus, said the troop deployment was an "unprecedented" offensive against the wave of dissent that has swept the country since the uprising began on March 15.
She reported checkpoints and heavy security in central Damascus.
Witnesses in Deraa told news agencies that at least five people were killed when assailants opened fire on a car.
The vehicle was riddled with bullets, a witness told AFP. Intense gunfire could be heard reverberating across the town, he said.
"We need international intervention. We need countries to help us," a witness in Deraa told the Associated Press by telephone.
"The minarets of the mosques are appealing for help. The security forces are entering houses. There is a curfew and they fire on those who leave their homes. They even shot at water tanks on roofs to deprive people of water."
Al Jazeera is unable to confirm the reported deaths.
Syria closed all border crossings on its southern frontier with Jordan as the military launched its operation in Deraa, Al Jazeera has learned from a security offical.
'Bodies lying on streets'
Thousands of soldiers swept into Deraa in the early hours of Monday, with tanks taking up positions in the town centre and snipers deploying on rooftops, witnesses said.
"Bodies are lying in the streets and we can't recover them," one activist said, explaining that they have little idea of the total number of casualties.
Al Jazeera's report showing video of shooting in the southern Deraa province on Sunday
Footage aired by an opposition news organisation on Monday, transmitted via satellite, appeared to show Syrian military firing at unseen targets with sniper rifles.
Al Jazeera is unable to verify the veracity of the footage.
"There are injured people. Scores have been arrested. The security are repeating the same pattern in all the centres of the democratic uprising. They want to put down the revolution using the utmost brutality," an unidentified rights campaigner in Damascus told Reuters.
In Jableh, where several protesters were gunned down on Sunday, witnesses said security forces in camouflage uniforms - some with their faces covered - and masked armed men dressed in black were roaming the town's streets.
"Jableh is surrounded by security forces," the witness said, speaking by telephone. "The dead are in the mosques and the houses. We can't get them out."
The Syrian Observatory for Human Rights, a Syrian rights group, said on Monday morning that at least 13 people had been killed in Jableh since Sunday's crackdown began.
The country has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it nearly impossible to get independent assessments.
Al Jazeera's Amin said that the events of Monday marked a change in methods by security forces.
Up until now, she said, security forces have cracked down in reaction to protests. But the flood of troops into Douma and Deraa had come in the absence of any demonstrations.
"Today, we're seeing a different tactic, with security forces sweeping the towns," she said, noting reports of house-to-house searches, arrests and random shooting coming from both towns.
Communications have been cut off and, for the first time, the military has become directly involved in quelling the uprising, much to the disappointment of opposition activists.
"They were hoping the army would not get involved," our correspondent said. "They feel this is only the beginning of a very serious crackdown."
One activist told Al Jazeera that some army officers had defected to fight alongside the people of Deraa against the government.
Two members stepped down from the provincial council in Deraa. The resignations came a day after two legislators and a religious leader from Deraa broke with the government in disgust over the killings.
Separately, Syrian intellectuals expressed their outrage over the violence, with a declaration on Monday signed by 102 writers and exiles from all the country's main sects.
"We condemn the violent, oppressive practices of the Syrian regime against the protesters and mourn the martyrs of the uprising," they said.
As Syria continued its violent crackdown, a US official told Reuters the administration was considering a range of options against the Assad government, including possible sanctions on senior officials.
The measures could include a freeze on assets and a ban on business dealings in the US, the official said. There was no immediate word on when such sanctions might be imposed.
Earlier, the UN's highest human-rights official called on Syria to rein in its security forces and investigate nearly 100 killings of protesters reported over the weekend.
Navi Pillay, the UN High Commissioner for Human Rights, denounced the escalation of violence in the country and called for detained activists and political prisoners to be released.
She said Syria has turned its back on international calls to "stop killing its own people".
"The first step now is to immediately halt the use of violence, then to conduct a full and independent investigation into the killings, including the alleged killing of military and security officers, and to bring the perpetrators to justice," Pillay said in a statement.
Source: Al Jazeera and agencies