Among the esteemed visitors were King Juan Carlos and Queen Sofia of Spain, who were meeting the Pope for the first time.
The royal couple had arrived for the morning ceremony at 9:40am local time and were greeted by the Archbishop of the Catholic Church, Georg Gänswein, who showed King Juan and Queen Sofia to their places.
Queen Paola and King Albert II of Belgium (left) and King Juan and Queen Sofia of Spain (right)
As the reigning couple took their seats to the right of the altar, the Spanish royals were joined by King Albert II and Queen Paola of Belgium, who abdicated last summer.
In keeping with the "Privilege of the White" tradition — the rule that says certain Catholic queens, princesses and duchesses are permitted to wear white attire and a mantilla during an audience with the Pope — the Spanish and Belgian queens were dressed in white.
Queen Sofia looked beautiful in her lace blouse and satin skirt, which she paired with a floral embroidered lace veil. Queen Paola looked equally as regal in her cream jacket and skirt combo, and white mantilla.
Keeping with tradition: Queen Sofia dressed in white
The day was a historic one in the history of Catholicism, as it was the first time that two Popes had been declared saints at the same time. On a more personal level, King Juan Carlos and Queen Sofia looked thrilled to be meeting the religious leader for the first time.
After the canonisation mass, the Spanish King was the first to be greeted by the Pope with a handshake. Queen Sofia then showed her respects by kissing the Pope's ring, a symbol of his pontifical power.
The couple spoke briefly with the Pope and will be treated to a private audience with the pontiff and his secretary of state Pietro Parolin on Monday morning.
King Juan and Queen Sofia of Spain meet Pope Francis for the first time
Other European royals who were spotted in attendance included the Grand Duchess Maria Teresa of Luxembourg, also wearing white, and her husband Henri, Grand Duke of Luxembourg.
The mass, which was co-celebrated by Pope Francis and his predecessor Benedict, 87, was watched by roughly one million pilgrims in the Vatican City, with thousands more tuning in on television and radio channels.
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Grand Duchess Maria Teresa of Luxembourg greets the Pope
"We declare and define as saints the blessed John XXIII and John Paul II," the Pope said during the ceremony, from an altar in front of St Peter's Basilica. Giant portraits of the two saints flanked the religious leader as he celebrated mass.
Pope Francis also paid tribute to Pope John Paul II and Pope John XXIII, saying, "They were priests, bishops and popes of the 20th Century. They lived through the tragic events of that century, but they were not overwhelmed by them. For them, God was more powerful."
After the ceremony, Pope Francis greeted crowds from his "popemobile", beaming widely and waving to his audience.