Syrian troops fired mortars into an area on the outskirts of Damascus today and hundreds moved in behind tanks to raid opposition districts and flush out rebels, activists said.
They said it was the first shelling inside the city limits since an uprising against president Bashar al-Assad began 16 months ago.
Video shot by opposition supporters showed black smoke rising from among trees and buildings.
There were no immediate reports of casualties.
"The raids have now started in the area. These are very dangerous for residents, they have to hide and there is no way they can move," said an activist named Bassam via Skype.
"We saw six armoured vehicles pass by in al-Lawan and two tanks heading in another direction, followed by three trucks filled with soldiers heading another way. There must be hundreds of soldiers."
Activists in the Kafar Souseh neighbourhood of the Syrian capital said Assad forces began firing mortar rounds into orchards on the outskirts of the city in the morning.
The al-Lawan and Basateen districts of Kafar Souseh form a semi-residential area of olive and fruit groves, lying astride a major interchange on the Hafez al-Assad southern bypass highway.
Tanks were firing from the big Hadi Mosque to the east of the fields, and from the al-Mazzeh military airport immediately to the west, according to activists.
"I woke up this morning and saw helicopters flying over the area. Then I started hearing the mortars. There were about six or seven of them in the past half hour. And now I just heard another one hit. We can see fire and smoke coming from one of the fields nearby," said anti-government activist Hazem al-Aqad.
"People are terrified, families are getting in their cars and rushing as fast as they can to other areas. About 200 people in my area have left so far," he told Reuters on Skype.
"This is a farm area, people have olive trees. It's a poor area, there have been a lot of protests," another activist source told Reuters.
"They believe the rebels are hiding in the fields so it seems they've started to launch an attack. This is the first time we've ever had shelling in an area within the capital."
Mr Bassam said there was "no sign of return fire" from the rebels.
"It will be hard for them to respond because of all the checkpoints and the army deployment."
Activist al-Aqad in Lawan said residents had seen hundreds of troops storming the area and setting up checkpoints on most streets.
The roadblocks on main junctions were backed by armoured vehicles or tanks, and arrests were being made.
At least 17,000 people have been killed during the uprising, according to the opposition's estimates. Dr Assad says he is defending his country against foreign-backed armed groups bent on overthrowing the government.
Reuters