Following the attack on the National Security Building in which top Syrian officials died, the Syrian army was reported to have “flooded” the streets of Damascus in a bid to crush rebel units.
Pictures from Syrian TV apparently showed the troops in the Midan neighbourhood while there have been reports of several major blasts around the city.
It is a major development in an uprising that has been going on for 17 months but that had, until recently, been confined to the provinces.
Earlier this week the Free Syrian Army launched what it called Operation Damascus Volcano, to try to seize control of the Syrian capital.
After the National Security Building blast, rebel spokesman Qassim Saadedine told a news agency by phone: “This is the volcano we talked about, we have just started.”