Syria's military said it halted operations at midnight, except against ISIS and other terror groups, state-run news agency SANA reported.
A Syrian activist group based in the UK said indications were there were pockets of violence, but large areas of the country were calm.
"Most areas in Syria are quiet since the start of the ceasefire, but violent clashes were reported less than two hours after it took effect in the Hama suburbs between regime forces and rebel fighters in Tal Beejo in northern Aleppo causing to regime forces to withdraw," said Rami Abdulrahman of the UK-based Syrian Observatory for Human Rights.
Neither SANA nor Russian state media has reported episodes of violence Friday morning.
Russian President Vladimir Putin on Thursday announced the agreement that the Syrian government and opposition rebels would cease fighting in the country's long-running civil war
According to Russian state media TASS, Putin said the two sides had also agreed to enter peace talks to end the conflict that has raged for nearly six years.
Turkish Foreign Minister Mevlut Cavusoglu said earlier that Russia and Turkey would be guarantors to the agreement.
Groups considered as terrorist organizations by the UN Security Council, such as ISIS, are excluded from the agreement, the Turkish Ministry of Foreign Affairs said.
Putin announced the deal at a meeting with Russian foreign and defense ministers. "Reports have just arrived that several hours ago there was a development that we all have looked and worked for so long," he said.
Three documents had been signed, Putin said: the ceasefire agreement, a package of measures to oversee the ceasefire and a declaration of readiness to enter into peace talks.
Putin said the "agreements reached are very fragile" and that they demanded "special attention and patience."
A statement from the Kremlin, carried by Russian news agencies, said Putin and Syrian President Bashar al-Assad discussed the agreement by telephone Thursday.
Assad said he would comply with it, according to the statement. Putin and Syria's President consider the ceasefire negotiations "an important step for a final solution of the crisis," the statement added.
A successful nationwide ceasefire hinges on many fighting factions laying down arms -- groups from Iraq, Iran and Lebanon are also fighting alongside the forces of Assad's regime.
In a news conference in Turkey's capital, the legal adviser for the Turkish-backed Free Syrian Army, Osama Abu Zeid, called the agreement "a complete truce with no exceptions or preconditions" that included "all areas and military factions in Syrian opposition territory."
"The main goal is improving the living conditions for Syrian citizens," he said.
"Our message to the Syrian population (is) that our fingers will remain on the trigger and we will present all sacrifices and we will take every path that will alleviate the suffering of the Syrian people."
Hadi Al Bahra of the Istanbul-based opposition Syrian National Coalition told CNN on Thursday that the ceasefire negotiations involved the military and humanitarian sides of the conflict, not Syria's political future.
"Assad is a big part of the problem; he doesn't have the capacity to really go forward with the peace agreement," he said of Syria's President.
The Syrian rebel group Ahrar al-Sham did not sign the ceasefire agreement, a spokesman said.
"Ahrar al-Sham has a number of reservations over the proposed agreement and the negotiations," the spokesman, Ahmad Qura Ali, said via Twitter. "We will make clear our reservations in due course."
Putin's announcement comes after several attempts at ceasefires by the international community crumbled.
It also follows several ceasefire agreements brokered by Turkey and Russia in the city of Aleppo this month. Most were broken, but a final one held and allowed the evacuations of tens of thousands of rebels and civilians from the city's east, which had been under the control of rebel groups for more than four years.
The Syrian regime then gained full control of Aleppo,
a major turning point that has limited the opposition's military and political options.
Turkey and Russia have differed, however, in their stance on Assad -- Turkish President Recep Tayyip Erdogan said last month that Turkish forces entered Syria to help end Assad's rule. Russia is Assad's most powerful ally and has propped up his regime since September 2015 with airstrikes.
Erdogan on Thursday said the ceasefire "opportunity in Syria shall not be missed."
"The parties in Syria will not expand the territories they have control over," he added.
Putin said Russia would begin scaling back its military presence in Syria, though it would continue to support the "legitimate Syrian government in its struggle with terrorism."
The UN Security Council has been widely criticized for failing to find a solution to the Syrian war.
Russia has shot down several resolutions on the conflict, using its veto power as a permanent member of the council.
A spokesperson for the UN Special Envoy to Syria Staffan de Mistura welcomed the ceasefire, saying in a statement that a cessation of hostilities was a "cornerstone" of a UN Security Council resolution from 2015 on Syria.
The statement said that de Mistura hoped the agreement would "save civilian lives, facilitate the delivery of humanitarian assistance across Syria, and pave the way for productive talks in Astana" in Kazakhstan.
But the statement also said that the developments should contribute to negotiations convened by the UN in planned Syria talks on February 8.
Turkey and Russia appear to be sidelining the United States, which has led an international coalition to fight ISIS in Syria and has vehemently opposed any attempt to keep Assad in power.
Turkey has worked with the US to fight ISIS in Syria, but it has worked more closely with Russia in recent months.
US State Department spokesman Mark Toner acknowledged that Washington was not part of the negotiations but said the agreement was "a positive development."
"An inclusive Syrian-led political process between the Syrian regime and the opposition is critical for establishing a durable settlement to this conflict," he said in a statement.
Russia has long accused the US of arming what it considers terrorist organizations, while Turkey has made similar claims in the past few days.
But Russian Foreign Minister Sergey Lavrov expressed hope Thursday that US President-elect Donald Trump would support Russia's direction on Syria's crisis settlement, Russian state media reported.
An estimated 400,000 people have been killed in Syria's brutal conflict -- a civil war that has drawn in several world powers.
Meanwhile, the US mission in Syria said Thursday that ISIS gang leader Abu Jandal al-Kuwaiti had been killed Monday in a coalition airstrike.
He was involved in the use of suicide vehicles, homemade bombs and chemical weapons against the Syrian Democratic Forces, the mission said in a statement.
He was a member of the militant group's war committee and was involved in ISIS' recapture of the ancient city of Palmyra
, the statement said.