Pope Francis on Sunday lashed out at what he termed "commercial war to sell arms" as he led mass prayers in Saint Peter's Square for Syria.

In his traditional weekly prayers in front of an unusually large crowd of faithful in the Vatican, the leader of the world's 1.2 billion catholics called for his followers to "say no to violence in all its forms, to the proliferation of weapons and their illegal trade."

In a departure from his prepared text, the pope asked what were the reasons for wars such as the one raging in Syria and mentioned "commercial war to sell arms."

"I invite you to continue to pray for the violence and devastation to stop! Let us work with a renewed commitment for a just solution to the internecine conflict," Francis told the crowd.

On Saturday, Francis led Catholics worldwide in a day of fasting and prayer for peace in Syria joined by Jews, Muslims and Orthodox Christians.