Syrian tanks and armoured vehicles swept into the coastal city of Latakia today and gunfire was heard in a district where thousands had protested against President Bashar al-Assad.
The deployment took place a day after security forces shot dead 20 people during nationwide marches in which demonstrators called for Assad's overthrow and vowed to "kneel only to God".
Assad's military crackdown on the five-month protests, in which activists say over 1,700 civilians have been killed, has prompted new US sanctions on Damascus and criticism from Arab states after months of regional silence.
The British-based Syrian Observatory for Human Rights said around 20 military vehicles deployed near the Ramle district of the Mediterranean city of Latakia, where 10,000 people had demonstrated on yesterday.
"Heavy gunfire could be heard from 10.30 am until midday," the group's head Rami Abdel Rahman said, adding that large numbers of people were fleeing the area.
He also reported that soldiers backed by loyalist gunmen, known as shabbiha, raided villages close to the northern town of Qusair near the Lebanon border, carrying out arrests.
Eleven people were killed by security forces in Qusair on Thursday, activists said, part of Assad's escalated military repression during the Muslim holy month of Ramadan which began two weeks ago.
Syrian authorities have barred most independent media, making it difficult to verify events on the ground in the unrest, one of a series of popular revolts against autocratic Arab leaders this year.