The Britain-based watchdog said hundreds of youths gathered late into the night on Tuesday in the upmarket district of Mazzeh, also in the capital, and chanted anti-regime slogans.

In Syria's commercial hub of Aleppo, at least two soldiers were killed when rebels attacked a checkpoint during the night, it said, adding that loud blasts were heard in the city on Wednesday.

The Observatory said 82 people were killed in violence across Syria on Tuesday: 30 civilians, 26 soldiers and 26 rebels.