Pope Francis invites an old friend from Argentina to ride with him in the popemobile (Photo: PA)

The Pope Francis effect continues as the pontiff showed on Wednesday that he will gladly tear up the rule book of papal protocol, inviting an old friend from Argentina to ride with him in his popemobile.

Father Fabian Baez, a parish priest from Buenos Aires, was standing among the crowds in St Peter's Square when Francis spotted him and signaled for the Vatican guards to help him jump the barricade and approach.

Francis then invited him to hop aboard, and Father Baez accompanied Francis on his tour through St. Peter's Square as the pope waved to well-wishers and kissed babies handed up to him.

Baez told the Associated Press: "The pope laughed and said: 'Come, sit down, sit down!' And he continued to greet the people and kiss babies. I was very moved."

A Vatican spokesman reportedly said the pope, the former archbishop of Buenos Aires, had described Baez as "a great confessor." Baez said he had known the 77-year-old since the 1990s.

He said he was shocked by Francis' invitation: "I said to myself  'What am I doing here? Mamma mia!'."