BEIRUT — Syrian children had little to celebrate on Sunday as Muslims marked the first day of the Eid religious festival in the grip of unrelenting violence.

"The children in the Old City district are sad because there are no sweets, no food, no gifts, no new clothes this Eid," said a young man from the city of Homs in central Syria who gave Abu Bilal as his name.

The rebel-held Old City district has been under siege by regime forces for more than two months, and rights watchdogs have warned that hundreds of families are trapped there.

"Today I went to pray in the only mosque still standing in the neighbourhood," Abu Bilal told AFP via Skype. "We put sandbags on the windows before we started praying, in case the army shelled the mosque."

Two children were among at least nine people killed in violence on the first day of Eid al-Fitr, the festival celebrated by Muslims across the world to mark the end of the holy month of Ramadan.

"A boy and a girl were killed by regime forces' shelling in the (rebel-held) town of Maaret al-Numan" in the northwestern province of Idlib, the Britain-based Syrian Observatory for Human Rights said.

According to the Observatory, violence has killed more than 23,000 people since March last year, including 1,300 children, while the UN puts the death toll at over 17,000 and says about 2.5 million are in need of help.

While President Bashar al-Assad made a rare public appearance with top officials for Eid prayers at a Damascus mosque, demonstrators took to the streets of the capital and other cities to vent their rage at the regime.

"Eid is here, Eid is here, God curse you, O Bashar al-Assad" protesters in the town of Qudsaya in Damascus province sang to the tune of Jingle Bells, according to amateur video posted on YouTube by activists.

In Damascus itself, Assad joined several top government and ruling Baath party officials at Eid prayers in Al-Hamad mosque.

"Syria will triumph against the Western-American plot being supported by the Wahhabis and takfiris (Sunni Muslim religious hardliners)," the imam Sheikh Mohammed Kheir Ghantus said, echoing the regime's long-standing rhetoric.

"With hardship, there is relief," he told worshippers, citing a verse from the Koran.

People also took to the streets in the northeastern Damascus district of Barzeh and several towns in Idlib, near the Turkish border, much of which is sympathetic to the 17-month-old revolt, according to the Observatory.

"This is how we celebrate Eid!" chanted a crowd of protesters who took to the streets of Kafr Zeita in the central province of Hama, according to amateur video posted on YouTube by activists.

Another video from Kafr Zeita showed a black-clad woman who has lost her children to violence stood before a silenced crowd as she wept: "May Assad's heart burn just like he burnt ours."