Choral music mixed with the sound of revving engines in the Vatican y as Pope Francis blessed crowds of motorcyclists in Rome for the 110th anniversary of Harley-Davidson.

Leather-clad bikers lifted their arms to greet the pontiff as he was driven up the avenue leading to St Peter's Square to celebrate mass and give his Sunday angelus. Some had "Papa Francesco" (Pope Francis) flags fixed to their bikes, which were parked all the way down the Via della Conciliazione among the trinket shops and snack stands.

Later, after celebrating a mass attended by men and women in Harley vests among the faithful, many of whom were involved in a two-day anti-abortion event, Francis greeted the "numerous participants" of the anniversary festivities. In a nod to the other large contingent in the crowd, he also offered prayers for "every human life, especially the most fragile, defenceless and threatened."

Emmanuele Jaconi, an Italian biker, said he saw nothing strange in the juxtaposition. "Just because we're motorcyclists doesn't mean we're not Catholics," he said.

Marcel Taschenhofer, who had come from Rostock in northern Germany, said he was not religious and was a little disappointed not to be able to ride his bike right into St Peter's Square. He added: "It's just a hobby. We like to drive our bikes around, and if the pope thinks it's good then all the better."

Rome was invaded by thousands of motorcyclists for the four-day celebration, during which Francis – in something of a marketing coup for the Wisconsin-based manufacturer – was given two white Harley-Davidsons for the Vatican police force, as well as a black leather vest with an eagle on the back.