The low rumbling of hundreds of Harleys is overtaking the buzz of scooters and cars in the streets surrounding the Vatican, and on Sunday it will even be present in St. Peter's Square.

The first Vatican-related Harley-Davidson event was with Pope Francis himself on Wednesday afternoon, following his weekly general audience. Accompanied by Archbishop Zygmunt Zimowski, the Pope received the unusual gift of a biker jacket and two of the famous motorcycles.

The City of Rome expects up to 35,000 Harley-Davidson aficionados to arrive for the 110th anniversary celebration of the iconic American street machines.

Although the celebrations will include a beach party in the nearby port town of Ostia and a music festival, there is also a spiritual aspect to some of the events.

This morning, for example, some bikers were able to participate in an 8:00 a.m. morning Mass in St. Peter's Basilica. Afterwards they gathered in the Pius XII Square, across the street from St. Peter's, and took in the sights.

On Saturday morning, the motorcycle enthusiasts will begin lining up for a massive parade from Ostia to downtown Rome.

The spiritual capstone of the anniversary celebration, which is the biggest in Europe, will be on Sunday, June 16, when Pope Francis blesses about 800 bikers and their rides in St. Peter's Square.

The crowd in the square for the blessing will be decidedly eclectic, since the Pope will be celebrating a Mass for pro-life movements right before he prays the Angelus and blesses the crowd.

The agreement to have the hundreds of Harleys in the square was arranged under Pope Benedict, but Pope Francis will surely have no trouble mingling with the diverse crowd.