Pope Francis on Saturday led an Easter vigil service, baptising eight adults, including a formerly undocumented Nigerian migrant beggar who became a hero when he disarmed an Italian thief wielding a cleaver.

The baptism took place during a long Holy Saturday, or Easter eve, Mass for some 10,000 people in St Peter's Basilica.

The church, the largest in Christendom, was dark at the start of the service before lights were turned on, signifying the passage from darkness to light when the Bible says Jesus rose from the dead.

The pope traditionally welcomes new members of the Church during the Saturday night service. This year, among those he baptised was John Ogah, 31, who Italian newspapers last year dubbed the 'migrant hero' and held up as an example of bravery and good citizenship.

Ogah was begging for change outside a supermarket in a Rome neighbourhood where many migrants live last September when he stopped a 37-year-old Italian who had just held up the store with a cleaver and was getting away with about 400 euros, according to the Catholic television station TV2000.

The Nigerian, who did not have permission to stay in Italy, held the man down until police arrived and then left the scene, fearing it would be discovered he did not have documents, according to La Repubblica newspaper.

Police using footage from surveillance cameras tracked him down and rewarded him by helping him get legal permission to stay in the country.

An Italian Carabinieri police captain who worked in the neighbourhood, Nunzio Carbone, was his godfather, or sponsor, at Saturday's baptism service.

Carbone and his fellow policemen helped Ogah get his immigration papers. The Nigerian now works as a stockman at a warehouse for a charity organisation.

The other newly baptised at the service came from Albania, Peru, Italy and the United States.

Francis has made defence of migrants a key part of his papacy.