Papal audiences usually entail peering at the pontiff through a sea of pilgrims' heads. For the lucky few, there might be a passing wave, a blessing for the sick or a kiss for a baby.

But Jorge Bergoglio proved once again 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 gestured for him to come on board.

Pope Francis greets Baez. Photograph: Tony Gentile/Reuters

Before he knew what was happening, Baez was sitting in the pontiff's white car, thinking – he told journalists afterwards – "What am I doing here? Mamma mia!"

He 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.