VATICAN CITY — Former Popes John Paul II and John XXIII were declared saints in the first double canonization of pontiffs since the Middle Ages in a ceremony that drew an estimated 800,000 pilgrims from around the world.

The canonization Mass at St. Peter’s Square, led by Pope Francis, started at 10 a.m. local time. Relics from the two saints, including a vial of blood from John Paul II and a piece of skin from John XXIII, were brought to the altar during the rites. Pope Emeritus Benedict XVI also attended.

In the past 700 years, only two other popes have become saints, Pius V in 1712 and Pius X in 1954, making today’s double celebration a unique event in the Roman Catholic Church’s recent history.

John Paul II, the church’s most traveled pontiff, becomes the fastest to be canonized since at least 1588, when Pope Sixtus V founded the Sacred Congregation for Rites to oversee the canonization process. During the Polish-born pope’s funeral on April, 2, 2005, mourners in Rome chanted “Santo Subito,” or “Sainthood Now.”

The Vatican heeded the call, waiving the traditional five- year waiting period before initiating the beatification process. Pope Francis last year described John Paul II as the “great missionary of the church” because he was a man “who proclaimed the Gospel everywhere.”

Marguerite Regnault, a 29-year-old psychologist from Paris, France, who traveled to Rome with a group of friends, said she was struck by his enthusiasm and how he transmitted it to others.

“I was here in the year 2000 as a teen,” she said. “I was here watching the giant screen at the Circus Maximus for the funeral, I returned again for the beatification sleeping in the square with other pilgrims, so in a way this is a circle that comes to a close and I’m here.”

Born Karol Jozef Wojtyla in Wadowice, Poland, he served as pope from 1978 to 2005. He used his position as the head of the world’s 1 billion Catholics to denounce totalitarianism and promote freedom and human rights. Poles credit him with helping hasten the fall of Communist regimes in the former Soviet bloc.

Dubbed “God’s Athlete” because of his love of sports such as skiing and mountain climbing, John Paul II traveled to more than 130 countries and visited every continent except Antarctica.

His final years were marred by revelations about the extent of sexual abuse carried out by priests. He survived an assassination attempt by Mehmet Ali Agca in 1981 and was beatified six years after his death by Benedict XVI, leaving him one step from sainthood and joining the list of those most venerated in the Catholic Church.

John XXIII, born Angelo Roncalli near the northern Italian city of Bergamo in 1881, is mainly known for convening in 1962 the Second Vatican Council, a gathering of bishops that played a key role in modernizing Catholicism by making changes such as allowing the use of vernacular in Masses and recognizing freedom of conscience and democracy.

Known as “Papa Buono,” or the “Good Pope,” for his down-to-earth personality, Roncalli worked as a papal diplomat in Eastern Europe, Turkey, Greece and France before serving as pope from 1958 until his death from cancer in 1963.

Many Italians recall Pope John XXIII’s impromptu 1962 “Speech to the Moon,” in which he greeted the Romans who were celebrating the opening of the Vatican Council, and asked them to pass on the Pope’s caress to their children.

He was beatified in 2000 by John Paul II, and Francis decided last year to fast-track his canonization even in the absence of a formally recognized second miracle usually required in order to proclaim someone a saint.

John Paul II is credited with curing a French nun with Parkinson’s disease, from which he also suffered, as well as a Costa Rican woman with an aneurysm in 2011. In the case of Pope John XXIII, the Vatican recognized as a miracle the 1966 healing of an Italian nun who was dying from a stomach hemorrhage.

The city of Rome has deployed thousands of police. Italian Interior Minister Angelino Alfano said yesterday that more than 120 delegations, including 24 heads of state, were expected to attend.

Giant television screens have been set up throughout the city to allow those who can’t fit into the square or the adjacent Via della Conciliazione, to view the proceedings.