One witness said large numbers of troops arrived the southern city of Daraa in busloads before dawn. Tanks later moved in as electricity, water and mobile phone service was cut.
An eyewitness counted 11 corpses, with another 14 lying in the streets, apparently dead or gravely injured.
All witnesses spoke on condition of anonymity for fear of reprisals. Syria has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it nearly impossible to get independent assessments.
The offensive into Daraa appeared to be part of new strategy for pre-emptive strikes against the opposition to President Bashar Assad's regime rather than reacting to marches and protests. Other crackdowns and sweeping arrests were reported on the outskirts of Damascus and the coastal town of Jableh.
But the assault on Daraa - an impoverished city on the Jordanian border - was by far the biggest in scope and firepower. Tanks fired volleys into the air and suspected opposition supporters were dragged from their homes along with their families, witnesses said.
More than 300 people have been killed across the country since the uprising began five weeks ago. But the relentless crackdowns have only served to embolden protesters, who started with calls for modest reforms but are now increasingly demanding Assad's downfall.
"We need international intervention. We need countries to help us," shouted a witness in Daraa.
Another witness said people were using mosque loudspeakers in Daraa to summon doctors to help the wounded as busloads of security forces and troops conducted house-to-house searches, causing panic in the streets.
"They are entering houses, they are searching the houses," he said. "They are carrying knives and guns."
The sweep into Daraa, a city of about 300,000 people, sought to hit the opposition movement at one of its pillars. The protests against Assad began there in March and several political figures from the area have stepped down to protest the violence in embarrassing defections from the regime.
The spark for the uprising was the arrest of some teenagers who scrawled anti-regime graffiti on a wall in Daraa.
In Geneva, the U.N. human rights chief, Navi Pillay, said Syria has turned its back on international calls to "stop killing its own people."
"Instead, the government's response has been erratic, with paper reforms followed by violent crackdowns on protesters ... The killings must stop immediately," Pillay said.