Fighting raged near a military base in Syria's north as a cease-fire in the bloody civil war began on Friday at dawn, activists said, illustrating the difficulty of enforcing even a limited truce coinciding with a Muslim holiday.
Elsewhere, violence appeared to die down, and thousands of protesters took advantage of the lull to mount some of the largest anti-government demonstrations in months.
The truce, proposed by UN-Arab League envoy Lakhdar Brahimi and endorsed by the Security Council, is set for only the four days of the Eid al-Adha holiday, has no monitoring mechanism and no stated plans for its aftermath.
The first serious disruption involved a radical Islamic group, Jabhat al-Nusra, which rejected the cease-fire from the outset. The group clashed on Friday with government forces for control of a military base outside of a strategic town on the road to the northern city of Aleppo, according to the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists.
Fierce fighting has been going on there for several days.
Opposition fighters seized Maaret al-Numan, which lies along the main highway between Aleppo and Damascus, earlier this month. Their presence has disrupted the ability of the Syrian army to send supplies and reinforcements to the northwest, where troops are bogged down in a stalemate with the rebels in Aleppo, Syria's largest city.
Activists said three people were killed in the shelling of the Damascus suburb of Harasta and two people died as a result of sniper fire. There were no reports of clashes or protests at the time of the attacks, the Observatory said.
The Observatory said protesters rallied after holiday prayers in Aleppo, the central province of Homs and the city of Hama. Demonstrators also took to the streets in the suburbs of Damascus, and across the southern province of Deraa, where the uprising began.
The demonstrations were reminiscent of the mass protests that ignited the civil war.
President Bashar Assad's government accepted the truce but declared it would respond to any rebel attack or attempts by foreign forces to intervene.
If the truce holds, it would be the first actual halt in 19 months of fighting that began with mass demonstrations but has transformed into a full-blown civil war with sectarian overtones and tens of thousands of dead.
Assad made a rare television appearance on Friday, attending morning prayers.
State television showed Assad, smiling and seeming relaxed, at an unidentified mosque in Damascus.
In a sermon at the prayers, imam Walid Abdel Haq called on Syrians to "stop quarreling because you are all brothers".
"Do you not see what has been happening for two years in the country, the destruction and death? Stop this," he said.
"Syrians must return to God to defeat this sectarian discord that they are trying to create in Syria. We must ourselves create a new Middle East and not the one trying to be imposed ... by the enemies of Islam and of Syria."
After the prayer, Assad was shown spending time with other worshippers, greeting and chatting with some of them.