Syrian government forces fired heavy machineguns at a main residential district of the city of Homs early on Saturday and residents said they saw military helicopters flying overhead.
The report of military activity in the western Syrian city came a day after multiple protests demanding the removal of President Bashar Assad.
Firing was concentrated on al-Khalidiya district, the activists said.
"The protests took a carnival-like atmosphere and the ante was raised a notch higher, hence we are seeing this response," an activist contacted in the city said.
He was referring to the displaying of shoes by protesters as a sign of defiance to the president.
The activists also reported that electricity and land phone lines in the city had been cut off.
Syrian authorities have banned independent media from Homs, the country's third largest city, and the rest of Syria, making it difficult to verify events on the ground.
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