The Free Syrian Army has declared the battle to "liberate" Damascus as having officially commenced amid heavy fire in the capital.

Tuesday's announcement came as the FSA claimed to have shot down a helicopter in Damascus.

The AFP news agency is reporting accounts of heavy machinegun fire in Sabaa Bahrat Square, where the regime of Bashar al-Assad has staged rallies to counter the opposition movement that began in March 2011.

At least 19 people were killed as tanks and helicopter gunships were deployed in Qaboon district and battles were fought in Al-Midan and Al-Hajar Al-Aswad, said the Syrian Observatory for Human Rights.