BEIRUT — A car bomb, shelling and gunbattles marred a tenuous cease-fire that went into effect in Syria on Friday, calling into question whether the four-day holiday truce brokered by the United Nations can endure.

The day started more calmly than usual, suggesting the government and the rebels were making some effort to keep their promises to give Syrians a respite from the violence engulfing the country for the duration of the Muslim Eid al-Adha holiday.

Many citizens took advantage of the morning lull to stage anti-government demonstrations around the country, surging onto the streets in a reminder of the initially peaceful start to the 19-month revolt against President Bashar Assad's rule.

But by nightfall, reports of violence began piling up. The Britain-based Syrian Observatory for Human Rights reported 70 deaths, a figure lower than the typical daily average of about 150 but far from encouraging, said Rami Abdelrahman, who runs the watchdog group in Britain. The name is his pseudonym.

"From early morning until midday, everything was fine, and after that everything changed," he said. "We are seeing clashes everywhere. Both sides are responsible."

In one of the worst incidents, a car bomb exploded outside a park where families had gathered to celebrate the holiday in the southern Damascus neighborhood of Daf al-Shouk, a poor and mostly Sunni area that has not previously been the scene of violence.

Five people were killed and 32 injured, according to state television.

Though bombings have become routine in Damascus, most target institutions of the state security services. Attacks that appear deliberately aimed at killing civilians are rare, and both sides accused the other of responsibility.

The official news agency SANA said Syrian security forces had responded to numerous attacks by "terrorist gangs" but that the government was otherwise upholding the cease-fire.

The Local Coordination Committees, an opposition activist group, accused the government of committing 110 breaches of the truce, including the shelling of civilian areas in Homs and Damascus in which civilians died.

In another incident, fighters with the extremist Jabhat al-Nusra organization attacked a military camp outside the strategic northern town of Maarat Numan, prompting government forces to shell a nearby village, according to the Syrian Observatory.

Al-Nusra, suspected of sympathies and perhaps ties to the wider al-Qaida franchise, had announced it would not abide by the cease-fire, and the attack illustrated the difficulty of bringing about any meaningful halt to the hostilities at a time the mainstream Free Syrian Army acknowledges it does not control all the rebel units battling the government.

Fighting also erupted in the strategic northern city of Aleppo between Kurdish fighters and Free Syrian Army rebels who had overrun the city's main Kurdish neighborhood of Ashrafiyeh the previous day, adding a new twist to the complexity of the battle for control of the country's commercial capital.