Pope Francis has urged Poland's leaders to overcome fear and show compassion to migrants at the start of his five-day visit to the country.
Fears run deep in the strongly Catholic nation that Muslim refugees could endanger the nation's security and erode its Christian traditions.
Francis, noting that many Poles have emigrated, spoke of the need to facilitate their return if any hope to repatriate, and understand the reasons that caused them to leave.
He added: "Also needed is a spirit of readiness to welcome those fleeing from wars and hunger, and solidarity with those deprived of their fundamental rights, including the right to profess one's faith in freedom and safety."
Last year Poles elected the right-wing Law and Justice party, which supports the Catholic church but has a strong anti-migrant message at odds with the pontiff's calls for mercy for people of other religions fleeing conflict.
Francis spoke in the southern Polish city of Krakow shortly after arriving at the airport and was welcomed by hundreds of singing and cheering people.
He was greeted by President Andrzej Duda and by first lady Agata Kornhauser-Duda and prime minister Beata Szydlo, who kneeled and kissed the papal ring in Poland's traditional way of greeting high Church officials.
He will join hundreds of thousands of young people from around the world for a major Catholic gathering.
Francis was also greeted by first lady Agata Kornhauser-Duda and prime minister Beata Szydlo, who kneeled and kissed the papal ring in Poland's traditional way of greeting high Church officials.
Mr Szydlo's conservative government openly declares attachment to the Catholic faith.
A military band played the anthems of the Vatican of Poland and the crowd waved the white-and-yellow flags of the Holy See and white-and-red flags of Poland.
Despite visibly tight security the pontiff waved from an open car window to the cheering crowd.
An official welcoming ceremony with speeches was to be held shortly later, at the royal Wawel Castle in Krakow.
It is the first time that Francis has visited an East European nation.
He earlier told reporters that the world is at war, but it is not a war of religions.
Francis spoke on the papal plane en route to Poland in the shadow of the slaying of an 85-year-old priest in France.
Asked about the killing, Francis replied: "It's war, we don't have to be afraid to say this."
He said that when he speaks of war, he is speaking of "a war of interests, for money, resources, dominion of peoples."
"I am not speaking of a war of religions. Religions don't want war. The others want war," Francis said.
The killing of the priest in a Normandy church on Tuesday added to security fears surrounding Francis' visit for the World Youth Day celebrations.
These concerns were already high due to a string of violent attacks in France and Germany.
Polish officials have deployed tens of thousands of security officials to cover the event.
Francis was greeted on arrival by the Polish Army band playing the anthems of the Vatican and of Poland.
He then travelled in an open car through the city, waving at crowds as he headed to the Wawel Castle for the main welcoming ceremony.
He is due to appear in the window of the residence of Krakow bishops, where he will be staying.
Francis will chat with some among the hundreds of thousands of pilgrims from around the world gathered for the World Youth Day celebrations running until Sunday.
"Let's live WYD (World Youth Day) in Krakow together!" the pontiff tweeted before departing from Rome, where he was bid farewell outside his Santa Marta residence by 15 refugees, new arrivals in Italy.
Groups of cheerful young pilgrims were seen in the streets of Krakow just hours before Francis' arrival.
Relics of St Mary Magdalene came to the St Casimir Church from France for the duration of World Youth Day, and were displayed in a case by the altar.
"Their presence helps us concentrate on our prayers and brings us closer to God," said Nounella Blanchedent, 22, from the French Caribbean island of Guadeloupe.
Ms Blanchedent was one of the volunteers helping with security and logistics at the packed church, where a Mass was held in French for pilgrims from France, Belgium and other countries.
Poland remains proud of the late pontiff, St John Paul II, who served as priest and archbishop in Krakow before becoming pope.
A sense of excitement was apparent in sunny Krakow on Wednesday 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," said souvenir shop owner Anna Gazda.
"It's difficult to move around even though offices have closed (for the event) and many people have left the city."