* Calm across Syria in early hours of ceasefire
* Shooting in Deraa to break up protest at mosque
* Army says will respond to any rebel attack
Syria's warring parties largely held their fire on Friday at the start of a four-day truce marking the Muslim holiday of Eid al-Adha, a short pause in hostilities which have killed 32,000 people and threaten to draw regional powers into a wider conflict.
President Bashar al-Assad's armed forces announced a conditional ceasefire on Thursday evening, responding to an appeal by international mediator Lakhdar Brahimi.
But they warned they would respond to any rebel attacks, or moves to exploit the truce to reinforce or resupply insurgents who are battling Assad's forces, including in Syria's biggest city Aleppo, and have seized swathes of territory from his grip.
A commander from the rebel Free Syrian Army said his fighters would also honour the ceasefire but demanded Assad meet rebel demands for the release of thousands of detainees.
Some Islamist fighters, including the Nusra Front, dismissed the truce before it even came into effect, but after a night of clashes in Aleppo, Damascus and the west of the country, activists said the country was largely calm on Friday.
"So far, since the ceasefire came into effect this morning, no shots have been fired except in the southern town of Inkhil," said Rami Abdulrahman, the head of the Syrian Observatory for Human Rights which monitors violence across the country.
Three people were wounded in Inkhil as they tried to protest after leaving a mosque where they had marked the start of Eid with special prayers. Several other protests in the southern Deraa province, cradle of the protests which erupted against Assad in March last year, were also broken up, Abdulrahman said.
Assad himself, who has vowed to defeat rebels he says are Islamist fighters backed by Syria's enemies abroad, was shown on state television attending Eid prayers at a Damascus mosque with senior officials.
Damascus residents said on Thursday night troops stationed on a mountain overlooking Damascus targeted Hajar al-Aswad, a poor district inhabited by refugees from the Israeli-occupied Golan Heights. There were no immediate reports of casualties.
"Consecutive artillery volleys from Qasioun shook my home," said Omar, an engineer who lives in al-Muhajereen district on a foothill of the mountain.
The fighting pits mainly Sunni Muslim rebels against Assad, from the Alawite faith which is linked to Shi'ite Islam, and threatens to draw in regional Sunni Muslim and Shi'ite powers and engulf the whole Middle East, Brahimi has warned.
"On the occasion of the blessed Eid al-Adha, the general command of the army and armed forces announces a halt to military operations on the territory of the Syrian Arab Republic, from Friday morning ... until Monday," an army statement read on state television said.
It reserved the right to respond if "the armed terrorist groups open fire on civilians and government forces, attack public and private properties, or use car bombs and explosives".
It would also respond to any reinforcement or re-supplying of rebel units, or smuggling of fighters from neighbouring countries "in violation of their international commitments to combat terrorism".
Qassem Saadeddine, head of the military council in Homs province and spokesman for the FSA joint command, said his fighters were committed to the truce.
"But we not allow the regime to reinforce its posts. We demand the release of the detainees, the regime should release them by tomorrow morning," he said.
Abu Moaz, spokesman for Ansar al-Islam, said the Islamist group doubted Assad's forces would observe the truce, though it might suspend operations if they did.
"We do not care about this truce. We are cautious. If the tanks are still there and the checkpoints are still there then what is the truce?" he said of the organisation, which includes several brigades fighting in the capital and Damascus province.
Brahimi's predecessor, former U.N. chief Kofi Annan, declared a ceasefire in Syria on April 12, but it soon became a dead letter, along with the rest of his six-point peace plan.
Violence has intensified since then, with daily death tolls compiled by opposition monitoring groups often exceeding 200.
UN SEES AID WINDOW
U.N. aid agencies have geared up to take advantage of any window of opportunity provided by a ceasefire to go to areas that have been difficult to reach due to fighting, a U.N. official in Geneva said.
"UN agencies have been preparing rapidly to scale up especially in areas that have been difficult to reach due to active conflict and which may become accessible as a result of these developments," he told Reuters.
The U.N. refugee agency UNHCR said that it had prepared emergency kits for distribution for up to 13,000 families - an estimated 65,000 people - in previously inaccessible areas including Homs and the northeastern city of Hassaka.
"We and our partners want to be in a position to move quickly if security allows over the next few days," UNHCR Syria Representative Tarik Kurdi in Damascus said in a statement.
The U.N. World Food Programme has identified 90,000 people in 21 hotspots from Aleppo to Homs and Latakia in need food parcels and will try to reach them through local agencies, the U.N. official said.