Pope Francis officially became the 266th leader of the Roman Catholic Church’s 1.2 billion worldwide followers yesterday during his formal installation Mass in St. Peter’s Square. Formally known as Cardinal Jorge Mario Bergoglio, the 76-year-old Argentinean is the first non-European pope in more than 1,200 years and the first from South America. During the Mass, which was attended by an estimated 150,000 to 200,000 people, Francis pledged to serve ‘‘the poorest, the weakest, the least important,’’ striking the same tones of humility that have marked the days since his election. — Lloyd Young ( 24 photos total)

Pope Francis waves to the crowd as he arrives in the popemobile for his inauguration Mass in St. Peter’s Square on March 19 in Vatican City, Vatican. The Mass is being held in front of an expected crowd of up to 200,000 pilgrims and faithful who filled the square to see the former Cardinal of Buenos Aires officially take up his role as pontiff. (Joe Raedle/Getty Images)