There were reports of isolated violence, including clashes in central Hama province between government forces and jihadist factions, a monitor said.
The Syrian Observatory for Human Rights said there were casualties among regime forces in the clashes after midnight with jihadists near the town of Mahardeh.
The fighters were believed to be from a faction that did not sign the ceasefire announced yesterday by Russian President Vladimir Putin, and confirmed by Syria's army and mainstream opposition bodies.
"There have not been any large violations," said Observatory director Rami Abdel Rahman.
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"From midnight until 8:00 am (0600GMT) there have been no civilian deaths recorded," he added.
AFP correspondents on the ground in rebel-held territory in northwest Idlib province and Eastern Ghouta outside Damascus also reported calm after the truce began.
The ceasefire is the first nationwide truce to be implemented in the country since September, and is intended to pave the way for new peace talks in Kazakhstan sponsored by Russia and Turkey.
And despite being left out of the process, Washington also hailed the truce agreement as a "positive development", saying it hoped it would bring new negotiations.
The agreement comes a week after the regime, which is backed by ally Russia, recaptured second city Aleppo in a major blow to rebel forces.
Putin said Damascus and the "main forces of the armed opposition" had inked a truce and a document expressing readiness to start peace talks.
Syria's conflict has become a complex multi-front battle, with a range of outside players intervening, including Russia, which launched a military campaign to bolster President Bashar al-Assad last year.
Putin said Friday he would now reduce Moscow's military contingent in Syria, though he added Russia would continue to fight "terrorism" and maintain its support for the government.
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