Polish pilgrims carrying the red and white flags of John Paul's beloved homeland were among the first to press into the square well before sunrise, held back by human chains of neon-vested civil protection workers trying to maintain order.
Most of those who arrived first had camped out overnight on air mattresses and sleeping pads along the side streets leading to the square. Others hadn't slept at all and took part in the all-night prayer vigils hosted at a dozen churches in downtown Rome.
''Four popes in one ceremony is a fantastic thing to see and to be at, because it is history being written in our sight,'' marveled one of the visiting Poles, David Halfar. ''It is wonderful to be a part in this and to live all of this.''
The Vatican on Saturday ended weeks of speculation and confirmed that retired Pope Benedict, 87, would indeed participate in the canonization. The move sets a remarkable precedent for the 2,000-year-old Catholic Church, which has never seen a reigning and retired pope celebrating a public Mass together.
Benedict had promised to remain ''hidden from the world'' after resigning last year, but Francis has coaxed him out of retirement and urged him to take part in the public life of the church.
Sunday's canonization is the first time two popes have been declared saints at the same time. Francis' decision to canonize two of the 20th century's greatest spiritual leaders amounts to a delicate balancing act, giving both the conservative and progressive wings of the church a new saint.
John, who reigned from 1958-1963, is a hero to liberal Catholics for having convened the Second Vatican Council. The meetings brought the church into the modern era by allowing Mass to be celebrated in local languages rather than Latin and by encouraging greater dialogue with people of other faiths, particularly Jews.
During his quarter-century papacy from 1978-2005, John Paul helped topple communism in Poland through his support of the Solidarity movement. His globe-trotting papacy and launch of the wildly popular World Youth Days invigorated a new generation of Catholics, while his defense of core church teaching heartened conservatives after the turbulent 1960s.
Kings, queens, presidents and prime ministers from more than 90 countries were expected to attend the canonizations. Some 20 Jewish leaders from the U.S., Israel, Italy, Francis' native Argentina and Poland were also taking part, in a clear sign of their appreciation for the great strides made in Catholic-Jewish relations under John, John Paul - and their successors celebrating the Mass.