Pope Francis condemned "unprecedented violence" by Islamist militants, saying at a Good Friday service that followers of religions who carried out acts of fundamentalism or terrorism were profaning God's name.

Security was heavier than usual at a traditional annual Via Crucis (Way of the Cross) candlelight procession around Rome's Colosseum, following the attacks in Brussels that killed at least 31 people. Police reinforcements arrived from other Italian cities, according to security sources.

In an emotional talk at the end of the procession, Francis condemned "expressions of fundamentalism (and) terrorist acts committed by followers of some religions which profane the name of God and which use the holy name to justify their unprecedented violence."

On Thursday he called the bomb attacks in Brussels that killed or wounded people from more than 40 countries a "gesture of war".

On Good Friday, Christians commemorate the day Jesus was crucified. "Stations of the Cross" rituals are held throughout the world recounting the 14 events between his condemnation by Pontius Pilate to his burial in a cave.

Francis also referred to the beheading and burning of Christians in the Middle East, and to those forced to leave their homes.

"O cross of Christ, today too we see you raised up in our sisters and brothers killed, burned alive, throats slit and decapitated by barbarous blades amid cowardly silence," he said, addressing thousands of people from the Palatine Hill.

"O cross of Christ, today too we see you in the faces of children, of women and people, worn out and fearful, who flee from war and violence and who often only find death and many (Pontius) Pilates who wash their hands," he said.

Francis condemned politicians who fuel conflict and "arms dealers who feed the cauldron of war with the innocent blood of our brothers and sisters, and give their children bread that has been dipped in blood to eat".

He said Europe's reaction to the migrant crisis had been blunted by "our indifferent and anaesthetized conscience" and decried unchecked destruction of the environment.

After the Brussels attacks, Italy intensified security at what the government has deemed "sensitive sites".

Islamic State militants have made threats against Catholic targets in Rome. Last year, a website used by militants ran a photo montage showing the movement's black flag flying from the obelisk at the center of St Peter's Square.

The pope leads an Easter vigil service on Saturday and on Easter Sunday he delivers a twice-yearly "Urbi et Orbi" (to the city and the world) blessing and message from the central balcony of St Peter's Basilica.