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.
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.