Syria's army has launched an attack on the southern city of Deraa, with tanks being used to support thousands of troops, according to activists.
One activist was quoted as saying that security forces were "firing in all directions", and there were unconfirmed reports of casualties, the BBC reports.
Deraa is the city in which protesters began calling for political reforms in March. Many are now demanding that President Bashar al-Assad step down after 32 years of rule.
News of the assault follows reports of crackdowns and arrests in other parts of the country.
A human rights campaigner told Reuters that security forces had also stormed the Damascus suburb of Douma on Monday, opening fire on civilians and arresting scores of people.
On Sunday, at least 13 people were reported to have been killed in the north-western city of Jabla.
Security forces are said to be rounding up suspected protest organisers despite the lifting of the state of emergency last week.
At least 350 people are reported to have been killed since March.
Foreign journalists have mostly been expelled from the country, making it impossible to verify the situation on the ground.
The New Zealand Ministry of Foreign Affairs and Trade is advising against all tourist and other non-essential travel to Syria.
The Australian government has issued a strong warning its citizens not to travel there and says Australians in the country should consider leaving.
Britain has also urged its nationals to get out of Syria.