It is the first major cessation of hostilities since 2011
A cessation of hostilities, described as Syria's best hope for peace in five years of fighting, appears to be holding, despite reports of violations on both sides.
Syrian state TV claims that residential areas in the capital Damascus have come under attack since the pause in fighting came into effect on the stroke of midnight local time.
Citing a military source, Syrian state TV said "terrorist groups" have fired rockets from neighbourhoods east of Damascus. No casualties were reported.
One rebel commander has also accused government forces of several violations.
Fares Bayoush, head of the Fursan al-Haqq rebel group which fights under the banner of the Free Syrian Army, warned that continuing breaches could lead to a collapse in the agreement.
The military has strenuously denied the allegations.
Brokered by the US and Russia, the deal marks the first major cessation of hostilities since 2011 when the civil war that has claimed more than 270,000 lives began.
It is designed to allow aid to reach civilians and to pave the way for peace talks to resume.
On Friday, US ambassador Samantha Power acknowledged there was "some scepticism" over whether the break in fighting would last, but said it offered the "best chance to reduce the violence".
Russia, which has been propping up the Syrian government with airstrikes, pledged to halt airstrikes for the day on Saturday to ensure no wrong targets are hit by mistake.
"...To avoid any possible mistakes when carrying out strikes, Russian military planes, including long-range aviation, are not carrying out any flights over Syrian territory on 27 February," the defence ministry said.
It came after Moscow was accused of bombing civilian positions near Damascus on Thursday night.
Beyond Saturday, Russia has vowed to keep up its bombing campaign against terrorist groups not covered by the deal, including so-called Islamic State and al Nusra Front.
Earlier, IS fighters reportedly stormed a northern border town captured by Kurdish fighters several months ago, prompting an outbreak of fighting.
The US-led coalition said it carried out 10 airstrikes in a bid to repel the assault, the Syrian Observatory for Human Rights reported.
IS also claimed responsibility for a car bomb attack east of Hama city on Saturday, which killed two people and wounded several others.
A second suicide blast in Hama province killed four people.
The UN Security Council gave its unanimous backing to the cessation of hostilities in a resolution drafted by the US and Russia.
It called on all the parties concerned to observe the terms of the agreement and "fulfil their commitments".
A previous agreement was due to come in to force last Friday but appeared not to have been observed by some groups.
On Sunday at least 140 people were killed in car bombings in Homs and Damascus.