The first major temporary truce in Syria's five-year civil war has come into effect.
The "cessation of hostilities" began at midnight (22:00 GMT Friday) with early reports saying front lines were silent.
However, Syrian state media said several shells had hit residential areas of the capital, Damascus. It did not say who had fired.
Low-level clashes have also been reported between rebel groups and government forces elsewhere.
A special taskforce chaired by the United States and Russia is to meet in Geneva to monitor violations of the agreement.
The truce involves Syrian government and rebel forces, but not the so-called Islamic State group (IS) or the al-Qaeda-linked Nusra Front.
In the run-up to the deadline, US President Barack Obama warned the Syrian government and its ally Russia "the world will be watching".
More than 250,000 Syrians have been killed in Syria's civil war and millions more have been forced from their homes.
'Halt to air strikes'
Russia halted air strikes at midnight on armed groups that had said they would abide by the temporary truce, a senior Russian defence official told reporters at a briefing in Moscow.
Russian warplanes have been grounded for the first day of the cessation to avoid hitting any wrong targets, military chief of staff Sergei Rudskoy said.
A few hours after the deadline passed, a car bomb killed two people outside the government-held town of Salamiyeh, near Hama, Syrian state media reported. IS is reported to have said it carried it out.
The temporary halt in fighting appeared to be holding on Saturday morning, reported the BBC's Mark Lowen from the Turkish town of Gaziantep, near the Syrian border.
Reuters news agency later reported claims from rebel groups of attacks from government forces.
A spokesman from the Jaish al-Islam group said two barrel bombs were dropped on its positions in the eastern Ghouta, near Damascus. The group said it also exchanged fire with government troops.
The First Coastal Division, part of the Free Syrian Army, says three fighters from an affiliated group, the Second Coastal Division, were killed in a ground attack in Latakia province.
The claims could not be verified.
A Syrian military source quoted by Reuters said the Syrian army "did not commit any breaches" against the rebels.
Further north, IS began an attack on the town of Tal Abyad, near the Turkish border, after midnight on Friday.
The US-led coalition launched air strikes to try to repel them, the UK-based Syrian Observatory for Human Rights said. It added that 45 IS militants were killed in the clash along with 20 Kurdish fighters.
Under the terms of the cessation of hostilities, military actions - including air strikes - would continue again IS and the Nusra Front by forces of the Syrian and Russian governments, and the US-led coalition.
UN special envoy for Syria Staffan de Mistura has said that peace talks will resume on 7 March if the truce "largely holds", adding that he had no doubt there would be "no shortage of attempts to undermine this process".
The cessation was brokered by the US and Russia, and is backed by a UN resolution. Previous talks in Geneva collapsed in early February after making no progress.
One of the key aims of the cessation is to allow desperately needed aid to reach people trapped in besieged areas.
The UN resolution names about 30 areas in dire need of aid, including eastern and western rural Aleppo and the eastern city of Deir al-Zour, which is under siege by IS.
Almost 100 rebel factions have agreed to respect the truce, the Syrian opposition umbrella group the High Negotiations Committee (HNC) says.
However, the HNC warned the Syrian government and its allies not to use the "proposed text to continue the hostile operations against the opposition factions under the excuse of fighting terrorism".
Russian President Vladimir Putin says his forces are targeting IS, Nusra Front and other extremist groups designated as legitimate targets by the UN Security Council.
However, over the course of its campaign in Syria, Russia has been widely accused of also attacking more moderate rebel groups fighting President Assad, an ally of the Kremlin.
Syria's civil war
Why is there a war in Syria?
Anti-government protests developed into a civil war that four years on has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran have pro-Assad forces on the ground, while a Western-led coalition and Russia are carrying out air strikes.