The deal, brokered by the United States and Russia, is the second attempt this year to bring the five-year civil war to an end, but it is unclear how widely it will be observed.
The tentative truce began at sunset, announced by the Syrian army which said a "regime of calm" would be applied across Syria.
After months of failed negotiations, a deal was finally struck between the United States and Russia on Friday.
The ceasefire, to be renewed every 48 hours, requires government forces and their allies, and rebel groups, to halt all attacks.
That includes aerial bombardments and attempts to gain additional territory at the expense of other parties.
Militant groups such as the self-proclaimed Islamic State, or Jabhat Fateh al-Sham - formerly known as al-Nusra Front - have not signed up.
US Secretary of State John Kerry says early signs are somewhat positive.
"The earliest reports are that there is some reduction in violence, as well as a few reports of fighting here and there. Though it is far too early to draw any definitive conclusions and I am not drawing any definitive conclusions."
Despite deep reservations and scepticism, rebel groups fighting to topple Syrian President Bashar al-Assad say they will respect the ceasefire.
But they believe it is skewed to favour the President.
The Syrian Kurdish YPG militia, which controls parts of northern Syria, will also halt offensive operations.
The Syrian army has promised to freeze its military operations for seven days, but warns it reserves the right to respond to any violation by what it calls "armed groups".
If the truce holds for seven days, the US and Russia will carry out coordinated air strikes on militant groups.
John Kerry says for that to happen, rebels must stop associating with al-Qaeda-linked militants and the Syrian government must allow aid into besieged towns.
"If there is no compliance with the cessation of hostilities, and no fulfillment of the principal of humanitarian access, then this arrangement, including the Joint Implementation Centre, will not go forward. Now I want to be clear as well, for all of the doubts that exist and we know there are many, there will be challenges in the days to come. We expect that, I expect that; I think everybody does. But despite that, this plan has a chance to work."
In the hours leading up to the ceasefire, Bashar al-Assad attended Eid prayers at a mosque in the Damascus suburb of Daraya, which had been evacuated by rebels and surrendered to government control last month.
The Syrian President gave a defiant message to opposition forces, indicating the fight against the rebels would continue.
"When we come to this area we give a message that the Syrian state is determined to recover every area from terrorists, re-establishing safety and security, rebuilding infrastructure, and rebuilding everything."
Aid agencies, desperate to reach countless besieged areas - particularly Aleppo - are cautiously optimistic the truce will hold.
A volunteer for Syrian rescue group the White Helmets says the situation in eastern Aleppo improved in the early stages of the truce.
"I consider that the situation is now quiet. I'm not hearing any sound of bombing, sound of helicopters, or bombing or fighting. The situation has changed, from today and yesterday. Before the ceasefire begins, there was a big or heavy bombing on the city, on all of the neighbourhoods in all of Aleppo city. One was of the neighbourhoods was targeted by six barrel bombs, and the situation is completely quiet in the east part of Aleppo city now."
The Syrian government is to refrain from flying combat missions anywhere where the opposition is present in agreed-upon areas.
But it remains unclear whether those areas include locations held jointly by rebels and Jabhat Fateh al-Sham.
Russia's Defence Ministry has also asked the Free Syria Army to stop fighting Kurdish units.
Russian General Staff's main operations command head, Sergei Rudskoy, says his country is ready to observe the ceasefire, but warned that strikes could still occur against certain targets.
"The Russian aerospace forces will continue hitting terrorist targets in Syria. To determine the targets for the Russian and American sides, a joint-executive centre is being set up which will make it possible to coordinate airstrikes by the Russian aerospace forces, and by aircraft from the US-led coalition."
Russia is a major backer of Assad, while the United States supports some of the rebel groups which oppose him.
More than a quarter of a million Syrians have been killed in the conflict and 11 million displaced in the world's worst refugee crisis.
All previous diplomatic initiatives have collapsed in failure.
John Kerry believes the situation is a toxic mix of interests and agendas.
"I have never seen a more complicated or entangled political and military, sectarian, somewhat religiously overtoned issue than what exists in Syria today. There are a bunch of wars going on. A bunch of different tensions between people. And you can cite them: whether it's Kurd and Turkey, or Kurd and Kurd, or Sunni-Shia or Assad versus (others) or others versus Assad, or countries that don't get along with each other in the region."
