Pope Francis on Sunday opened a giant open-air mass, the final religious service of his triumphal tour of the United States, attended by an enormous crowd in downtown Philadelphia.

Up to 1.5 million people from across the world and around America were expected to attend the religious service on the Benjamin Franklin Parkway before the pope boards a flight back to Rome.

The mass, held outside the Philadelphia Museum of Art is open to the general public and brings to a close the World Meeting of Families, a Catholic event that happens once every three years.

The Benjamin Franklin Parkway stretches around a mile (1.6 kilometers) long and television footage showed the crowd spread across much of it, but there was no immediate confirmation of how many people attended.

Draconian security measures have been in place the entire weekend in Philadelphia, with traffic banned from the downtown area and members of the public forced to travel by foot.