Syria was calm early Friday after the country's army and main rebel force said they had agreed to a ceasefire for the four-day Muslim holiday of Eid al-Adha, the Syrian Observatory for Human Rights said. The calm had settled following a night of heavy fighting across the country, the watchdog group said. Syrian state television showed President Bashar al-Assad attending morning prayers for the start of Eid at a mosque in Damascus, smiling and seeming relaxed as he chatted with other worshippers. The Observatory said fighting had taken place overnight around the country, including in the Damascus area, in Syria's commercial capital Aleppo and in the centre of the country near the border with Lebanon. The ceasefire comes at the initiative of UN and Arab League peace envoy Lakhdar Brahimi. Both the regime and the main rebel force, the Free Syrian Army, have agreed to temporarily lay down their arms but reserved the right to respond to any aggression. If the ceasefire holds, it would be the first real breakthrough in halting -- even temporarily -- the 19-month conflict that rights groups say has killed more than 35,000 people in Syria.