The Vatican estimated about 100,000 took part in the Rome event yesterday, making it one of the largest rallies in the West against proposed US-led military action against the Syrian regime following the Aug 21 chemical weapons attack near Damascus.
Francis spent most of the vigil in silent prayer, but during his speech he issued a heartfelt plea for peace, denouncing those who are "captivated by the idols of dominion and power" and destroy God's creation through war.
"May the noise of weapons cease!" he said. "War always marks the failure of peace, it is always a defeat for humanity."
In Damascus, a few dozen Syrian Christians attended a service in the al-Zaytoun Church, joining Francis' invitation for a global participation in the day of fasting and prayer and to oppose outside military intervention in the conflict.
Greek Catholic Patriarch Gregorios III Laham of Antioch and All East presided, saying most countries supported a political solution to the crisis in Syria and few wanted military action.
Francis announced the day of fasting and prayer September 1, alarmed at the acceleration of US threats to strike Syria after the chemical weapons attack.
Since then, the Vatican has ramped up its peace message, summoning ambassadors for a briefing by the Holy See foreign minister this week.
Francis appealed directly to world powers at the Group of 20 meeting in Russia, urging them to abandon the "futile pursuit" of a military solution in Syria and work instead for a negotiated settlement.
Vatican Radio reported similar initiatives were taking place throughout Italy, in Cuba and elsewhere. Even the grand mufti of Damascus, who thanked the pope for his initiative in a letter earlier this week, invited Muslims to join the fast in solidarity.
