"It's the bloodiest day since the start of the ceasefire (on April 12) and one of the bloodiest since the start of the revolt against the Syrian regime," Observatory director Rami Abdel Rahman told AFP.

The day's violence had killed at least 104 civilians, 54 soldiers and 10 rebel fighters, according to the toll compiled by the London-based organisation, based on the reports of activists inside the country.

The heaviest losses were in the central region of Homs, where 31 civilians and a rebel fighter died; and in the opposition bastion of Douma, near Damascus, where 30 civilians died.

In the southern region of Deraa, 24 civilians -- including two children -- and five rebels were killed.

Those killed elsewhere in Syria included a young girl, who died in the northwestern city of Idlib, the Observatory added.

More than 15,000 people have been killed since the revolt against Syrian President Bashar al-Assad's regime began in March 2011, according to the Observatory.