Francis led thousands of people in St Peter's Square in prayer today, saying he was "close to the relatives and all those who are weeping for the dead."
Britain is still looking for other suspects in the May 22 bombing at an Ariana Grande concert that killed 22 people.
Police have 11 suspects in custody but are looking for other members of the network of attacker Salman Abedi, a 22- year-old Briton of Libyan descent
The Manchester appeal was Francis' second prayer of the day: He also denounced the "act of ferocious violence" against a bus full of Coptic Christians in Egypt that left 29 dead Friday; the Islamic State group claimed responsibility.