The world is at war but it is not a war of religions, Pope Francis has said.

Francis spoke to reporters on the papal plane en route to Poland from Rome, who asked him about the slaying of an 85-year-old priest in a Normandy church on Tuesday.

Francis replied: "The real word is war...yes, it's war. This holy priest died at the very moment he was offering a prayer for all the church."

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He went on: "I only want to clarify, when I speak of war, I am really speaking of war ... 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."

More than 39,000 police and other security officers will be ensuring the safety of Francis' meeting with hundreds of thousands of young pilgrims from around the world in southern Poland.

Security concerns were heightened after the Catholic priest was killed in a knife attack claimed by the Islamic State group.

The Herald:

Francis was due to arrive in Krakow for a visit lasting until Sunday during which he will join young Catholics for World Youth Day.

The visit includes open air Masses and prayers with some 1.5 million participants of World Youth Day, and visits to the site of the former German Nazi death camp of Auschwitz-Birkenau and to Poland's holiest shrine of Jasna Gora.

"We have sent on to the Polish streets more than 7,500 officers who will be there providing security each day at railways stations, airports and shopping centres," said Mariusz Blaszczak, Poland's interior minister.

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Mr Blaszczak appealed to people to be vigilant and to report any unusual or worrying situations to police or other security officials.

Pope Francis will meet with young Catholics and visit Auschwitz as well as some of Poland's most important Catholic shrines.

This will be the first visit by Francis to Poland, one of Europe's most deeply Catholic nations and one still devoted to the memory of the late Polish pope, St John Paul II.

One youngster waiting for the pope told of her horror at the killing in Normandy.

"It shocked me because it seems they waited for the time of World Youth Day to attack us Catholics," said Nounella Blanchedent, 22, from the French Caribbean island of Guadeloupe.

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"They chose the time of such a mass event to gather more attention to what they do. It's a pity because all that we want to do is to be together, sing and praise God. And they want to disturb that."

She was one of the volunteers helping with logistics at the packed St Casimir Church, where a Mass was being held in French for pilgrims from France, Belgium and other countries.

Relics of St Mary Magdalene came to the church from France for the duration of World Youth Day.

Pope Francis is arriving on his first visit to Poland, a predominantly Catholic country that is still proud of the late pontiff, St John Paul II, who served as priest and archbishop in Krakow before becoming pope.

The sense of expectation was apparent in Krakow with papal white-and-yellow flags and images of Francis and John Paul II decorating the streets.

Stages were put up at many locations for concerts and other activities that are being held by and for the pilgrims in Krakow.

There was a heavy presence of police and other security forces across the city, as crowds were increasing everywhere.

"I have never seen so many people in Krakow.

"It's difficult to move around even though offices have closed (for the event) and many people have left the city," said Anna Gazda, 43, owner of a souvenir shop.

Some 200,000 pilgrims attended an inaugural Mass on Tuesday afternoon.