Syrian government tanks and troops have gone on the offensive in the southern town of Deraa, the cradle of the protests which began last month.
Residents say the forces moved in just after dawn prayers, opened fire at random, while bodies were left lying in the street.
The military action is evidence, say rights groups of the escalating crackdown in cities across the country.
Syria has now closed all its land border crossings with neighbouring Jordan in what one official said, ‘related to what appears to be a major security operation’.
In Douma a suburb of Damascus campaigners say forces stormed through the streets while many people were arrested.
It is one of three cities which have been attacked claims a leading Syrian human rights activist. In a statement she said it is, ‘a savage war designed to annihilate Syria’s democrats’.
Over a hundred people are reported to have been killed since the start of the weekend.
The action comes despite President Bashar al-Assad ending the 48-year state of emergency in the country.
An amateur video uploaded to a social website on Monday showed a crowd of Syrian men carrying a young girl with severe injuries through the streets of what is reportedly the Barzah district of Damascus.
The campaign group Human Rights Watch has called for a UN inquiry and international sanctions following the killings at the weekend.