BEIRUT (AP) — Fighting raged near a military base in Syria’s north as a cease-fire in the bloody civil war was supposed to go into effect today 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 today 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.

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.

The latest fighting showed the complexity of the situation, with the badly fragmented opposition sending mixed signals about the truce, some endorsing it but others rejecting it as irrelevant.

President Bashar Assad’s government accepted the truce but left significant loopholes.