There were reports of violence and deaths in some areas.
Ahmad Fawzi, a spokesman for U.N.-Arab League envoy Kofi Annan, said an advance team of up to 12 observers would enter Syria to monitor the truce after the U.N. Security Council voted to send them, possibly Friday, The New York Times reported.
The contingent of observers would be followed by a mission of 250 to be sent later, Fawzi said. That mission would need Syrian approval.
Some observers expressed doubts about whether the cease-fire would hold.
"I do not believe in [Syrian President] Bashar Assad's sincerity, nor, unfortunately, in the cease-fire," French President Nicolas Sarkozy told a television interviewer,.
The Britain-based Syrian Observatory for Human Rights reported 11 people had died in clashes. Tanks, armed checkpoints and rooftop snipers could be seen in areas that have been beset by violence.
At least one army officer was killed and more than two dozen wounded by a roadside bombing near Aleppo, Syria's state-run SANA news agency said, blaming armed terrorist groups.
At the United Nations, Syrian Ambassador Bashar Jaafari told reporters: "The Syrian government has ended the violence on its side. We expect those who have influence over the armed groups to do the same."
A rebel group said, activists vowed mass protests to test the truce's strength.
Low-flying aircraft swept over the Inshaat and Baba Amr former opposition-stronghold neighborhoods of the central-western opposition stronghold of Homs, with shelling occurring in other parts of the city, the Local Coordination Committees of Syria reported early Friday.
"Three powerful explosions" shook the northeastern city of Deir Ezzor, and explosions and heavy gunfire were reported in several Damascus suburbs, including Darayya, a small city northeast of the capital previously attacked by regime artillery, the coordination committees said in a report monitored by United Press International.
"Indiscriminate gunfire" came from at least one security checkpoint in the northwestern Idlib province, near Turkey, an area previously targeted by the regime for hosting large numbers of armed groups, the committees report said.
The reported military action could not be independently confirmed because the Assad regime severely restricts international journalists in Syria.
The coordination committees did not say if the opposition Free Syrian Army had responded to the alleged attacks.
The rebel army, made up largely of Syrian army defectors, had promised to counter any regime violations of the cease-fire by forces loyal to the regime.
The regime and opposition Syrian National Council had no immediate comment on the coordination committees report, nor did the U.S. State Department or Annan, who brokered the cease-fire.
The council earlier called on activists to test the Assad government's commitment to the cease-fire by staging mass protests across the country following Friday prayers, the main time of Arab Spring protests, including in Syria before the Assad regime's brutal crackdown brought them to an end.
The U.N.-backed cease-fire agreement called for the regime and rebel forces to end hostilities by 6 a.m. local time Thursday. Shortly after the deadline, calm was reported across the country's conflict hot spots, but sporadic outbreaks were reported later, with each side blaming the other for the violation.