An open-topped popemobile carrying Pope Leo XIV drove on Sunday morning through St Peter's Square at the Vatican, where thousands have gathered ahead of the newly elected American pontiff's inaugural mass.

The mass, called the Holy Mass for the Beginning of the Pontificate, will begin in Vatican City on Sunday at 10 a.m. local time, the Vatican Press Office said.

This is a developing story. Please check back for updates.

ABC News' Megan Forrester contributed to this report.