Syria's military announced at 7 pm (1600 GMT) that it would abide by a cease-fire until Sunday at midnight, while maintaining its right to defend itself against any violations.
The cease-fire marks the latest attempt to end the five-year conflict, which has killed more than 250,000 people and driven some 11 million from their homes.
The 2011 uprising against President Bashar Assad began with peaceful protests but escalated into a civil war following a brutal government crackdown and the rise of an armed insurgency.
Russia's Deputy Foreign Minister Mikhail Bogdanov said peace talks between opposition groups and the government could resume as early as next month.
Ahmad al-Masalmeh, an opposition activist in the contested city of Daraa, said calm had prevailed over the city since 4 pm, but observers elsewhere in the country reported fighting all the way up to and after the start of the cease-fire.
The terms of the agreement permit government forces to target the Al-Qaida-linked Jabhat Fatah al-Sham for the first week of the cease-fire. It was unclear whether the group's positions were being targeted after the truce began.
Hours before the cease-fire went into effect, Assad vowed that his government would take back land from "terrorists" and rebuild the country.
But in the buildup to the start of the truce, government forces and their allies bombed opposition areas in the north, while al-Qaida-linked militants pushed on with an offensive in the south.
In Geneva, the UN envoy for Syria said his office would monitor the start of the cease-fire "carefully, before making any hurried comments."
Staffan de Mistura said in a text message to The Associated Press on Monday that no statement from his office about the truce was expected before the following afternoon.
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