The crowd was so large it stretched back along all of Via della Conciliazione, the half kilometre-long, broad boulevard that starts at the Tiber River. Even its bridges were packed with pilgrims.
The Mass, which began under a light rain, was also attended by former Pope Benedict, who last year became the first pontiff in six centuries to step down.
Benedict walked with a cane and was dressed in white vestments. His attendance gave the ceremony a somewhat surreal atmosphere created by the presence of reigning pope, a retired pope and two dead popes buried in the basilica behind the altar.
John XXIII, who reigned from 1958 to 1963 and called the modernising Second Vatican Council, and John Paul II, the Pole who reigned for nearly 27 years, played a leading role on the world stage.
The overwhelming majority in the crowd were Poles who had travelled from their home country and immigrant communities as far afield as Chicago and Sydney to watch their most famous native son become a saint.
Hundreds of red and white Polish flags filled the square and the streets surrounding the Vatican, which were strewn with sleeping bags, backpacks and folding chairs. It was one of the biggest crowds since John Paul's beatification in 2011.
"For years Pope John Paul II took the Church to the ends of the earth and today the ends of the earth have come back here," said Father Tom Rosica, head of Canada's Salt and Light Catholic television network.