BEIRUT (AP)—Fighting raged near a military base in Syria's north as a cease-fire in the bloody civil war was set begin 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-regime demonstrations in months.

The truce, proposed by U.N.-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, that rejected the cease-fire from the outset. The group clashed Friday with regime 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.