ROME - Pope Leo led a candle-lit service with thousands of Catholics inside Rome's famed Colosseum on Friday evening that featured prayers for war orphans and deported immigrant children, and a warning to world leaders that their decisions will be judged by God one day.

The pope, who has emerged as an outspoken critic of the Iran war, listened as an evocative set of spiritual meditations was read aloud inside the ancient amphitheatre to observe Good Friday, the day Christians mark Jesus' death by crucifixion.

"Every person in authority will have to answer to God for the way they exercise their power," said the first meditation. "The power to start or end a war; the power to instil violence or peace."

Leo, the first U.S. pope and at age 70 a relatively young pontiff, carried a tall wooden cross during a traditional procession known as a Via Crucis (Way of the Cross), which marks 14 of the final moments in Jesus' life, from his sentencing to death up to his burial.

The pope stopped at various points inside the Colosseum to hear Bible readings, writings from St. Francis of Assisi, and the spiritual meditations, which focused extensively on social justice issues.

The spiritual texts, written by an Italian priest chosen for the task by the pope, did not name any specific world leaders.

Thousands of Catholics taking part inside and on surrounding cobblestone streets prayed at various points for refugees, victims of human trafficking, political prisoners and "those who have died beneath the rubble" during the world's conflicts.

They also recognised children who were imprisoned during protests or "deported by policies devoid of compassion", without referencing specific countries.

Leo has previously criticized U.S. President Donald Trump's hardline immigration policies, questioning whether they are in line with the Church's pro-life teachings.

Good Friday is the second of four Catholic holidays leading to Easter Sunday, when Leo will deliver a special blessing and message from the balcony of St. Peter's ​Basilica.

One of the most closely watched appointments on the Vatican's calendar, the Easter speech is usually a time when the pope makes a major international appeal.

Reporting by Joshua McElwee; Editing by Edmund Klamann)