Abu Osama al-Midani, an opponent of President Bashar Assad, told CNN the bomb attack killed at least 20 people. He said the bomb went off in a public square in an anti-government stronghold in the capital.
Other witnesses put the number of deaths at 10 to 20.
Midani said the square had been converted into a temporary playground for families celebrating the four-day Eid, one of the major Muslim religious holidays.
The government blamed dissident rebels for the bombing, the Los Angeles Times reported.
The Syrian Observatory for Human Rights said rebels and government forces fought in Idlib province after morning prayers, Voice of America said.
The two sides had agreed the prayers would mark the start of the temporary truce.
The Syrian army and rebel leaders had said they would observe the cease-fire but would respond to any attacks. The Syrian army warned its neighbors against granting safe passage to rebel fighters.
The temporary break was negotiated by U.N.-Arab League special envoy Lakhdar Brahimi.
Activists reported three people were wounded Friday when security forces fired on protesters in southern Daraa province.
Government-run television showed Assad mingling with worshippers attending Friday's prayers at the beginning of the Muslim holiday.
Agencies of the United Nations, hoping the cease-fire will be honored, have been making plans to deliver humanitarian aid, The New York Times reported. Brahimi had stressed that delivering aid to cities such as Aleppo, Homs and Idlib was a key component of the truce.
One rebel commander in the north, Arafat Mahmoud, said he suspected the Syrian Army was trying to regroup.
"The regime is looking for an exit to get ammunition and food supplies," he told the Times via Skype. "All the regime's tricks have been revealed, they want a cease-fire just to reinforce their bases."
In Saudi Arabia, government officials moved to expel three employees from the Syrian Consulate in Jeddah, Voice of America said. A Saudi Foreign Ministry statement said the three were told to leave for activities "incompatible" with their work, but provided no other details.
Saudi Arabia closed its embassy in Damascus in March, several weeks after it expelled Syria's ambassador.
At least 106 people were killed across the country Thursday, the opposition Local Coordination Committees of Syria reported. The victims included 36 people in Aleppo and 33 in Damascus, Syria's two largest cities, the opposition said.
In Washington, State Department spokeswoman Victoria Nuland said: "What we are hoping and expecting is that they will not just talk the talk of cease-fire, but that they will walk the walk, beginning with the regime. And we will be watching very closely."