Pope Francis is in Poland to preside at a gathering of more than a million Catholic youth from around the world, sometimes called the "Catholic Woodstock".

However, the usually joyous event is being held in the aftermath the murder of a priest in a church in Normandy, France. The killing of 85-year-old Father Jacques Hamel, forced to his knees by Islamist militants who then slit his throat, has added to security fears surrounding the Pope's visit for World Youth Day.

World Youth Day Krakow Poland
Pope Francis waves as he is welcomed by Polish President Andrzej Duda and his wife Agata Kornhauser-Duda at Balice airport near KrakowDavid W Cerny/Reuters
World Youth Day Krakow Poland
People in traditional outfits wait for the arrival of Pope Francis at John Paul II International airport in Krakow-BaliceJoe Klamar/AFP
World Youth Day Krakow Poland
Pope Francis is greeted as he is driven from Balice military airport into KrakowJoe Klamar/AFP
World Youth Day Krakow Poland
Pope Francis is welcomed by the faithful as he travels to Wawel Royal Castle in KrakowKacper Pempel/Reuters

Thousands of young Catholics from around the world are gathered in Krakow for the festival. Over 350,000 people from 187 countries have registered online but organisers expect up to 1.5 million people to attend the main events, which include a night vigil with the pope on Saturday (30 July) and an open-air mass celebrated by the pontiff on Sunday.

World Youth Day Krakow Poland
Pilgrims dance during World Youth Day in KrakowAgencja Gazeta/Michal Lepecki/Reuters
World Youth Day Krakow Poland
Pilgrims covered in rain coats play on World Youth Day at the Main Square in KrakowAgencja Gazeta/Mateusz Skwarczek/Reuters
World Youth Day Krakow Poland
Pilgrims wearing raincoats pray before the opening ceremony of World Youth Day in KrakowDavid W Cerny/Reuters
World Youth Day Krakow Poland
Despite the heavy rain, pilgrims gather on the Blonia Meadows in Krakow, to celebrate the Opening MassWojtek Radwanski/AFP
World Youth Day Krakow Poland
A pilgrim waits for the opening ceremony of World Youth Day in KrakowDavid W Cerny/Reuters
World Youth Day Krakow Poland
Pilgrims sing songs at the main square during World Youth Day in KrakowDavid W Cerny/Reuters
World Youth Day Krakow Poland
A pilgrim prays during the World Youth Day Opening MassJoe Klamar/AFP
World Youth Day Krakow Poland
A priest gives a holy communion to a pilgrim during the Opening Mass in KrakowJoe Klamar/AFP
World Youth Day Krakow Poland
Pilgrims from Brazil walk the streets of Krakow after the Opening MassBartosz Siedlik/AFP
World Youth Day Krakow Poland
Pilgrims pray during the Opening Mass for World Youth DayJanek Skarzynski/AFP
World Youth Day Krakow Poland
Pilgrims from all over the world celebrate after the Opening Mass of World Youth DayJanek Skarzynski/AFP
World Youth Day Krakow Poland
Young people and a monk play football in front of the main railway station in KrakowJanek Skarzynski/AFP
World Youth Day Krakow Poland
Young Catholics and a nun from France dance on the market square in KrakowJanek Skarzynski/AFP
World Youth Day Krakow Poland
Young people from around the world gather at the market square in KrakowJanek Skarzynski/AFP

World Youth Day was established in 1985 by Polish-born Pope John Paul II, whom Francis declared a saint in 2014, and aims to inspire young people to follow Christian values of peace and love in life. The gatherings are held every two or three years. The first meeting was held in Rome in 1986, attended by John Paul II and some 30,000 participants. The largest World Youth Day gathering was in the Philippines in 1995, when an estimated five million people attended a Mass celebrated by John Paul II.

World Youth Day Krakow Poland
A woman carries a Pope Francis puppet near Cracovia Stadium in KrakowAgencja Gazeta/Jakub Porzycki/Reuters
World Youth Day Krakow Poland
A poster with Pope Francis is seen near Cracovia Stadium in KrakowAgencja Gazeta/Jakub Porzycki/Reuters
World Youth Day Krakow Poland
A policeman stands guard during the Opening Mass in KrakowJoe Klamar/AFP
World Youth Day Krakow Poland
A pilgrim poses next to a picture of Pope Francis at the main square during World Youth Day in KrakowDavid W Cerny/Reuters
World Youth Day Krakow Poland
A pilgrim watches a mass on a large screen in KrakowJoe Klamar/AFP
World Youth Day Krakow Poland
A priest listens to a confession on the first day of the World Youth Days in KrakowJoe Klamar/AFP
World Youth Day Krakow Poland
A view of the altar during the Opening Mass in KrakowJanek Skarzynski/AFP

Security was already tight after a string of violent attacks in France and Germany. Polish officials say they have deployed tens of thousands of security officials to cover the event. Francis is expected to use the event to pay tribute to the murdered priest, who some say should be declared a martyr because of the way he was killed for his faith.

Pope Francis said the murder was particularly "barbarous" because it happened during the central rite of Christian worship as the priest was saying morning Mass. He spoke to reporters on the papal plane en route from Rome to Poland. Asked about the slaying of the priest, Francis replied: "It's war, we don't have to be afraid to say this." He clarified that when he speaks of war, he is speaking of "a war of interests, for money, resources. ... I am not speaking of a war of religions. Religions don't want war. The others want war."

During his five-day visit to Poland, the Pope will visit the Nazi death camp at Auschwitz-Birkenau, and pray at Czestochowa, site of the country's holiest shrine.