Francis said Jesus wants the church "to be a church on the move, a church that goes out into the world", as he gave his homily in a new monumental church dedicated to John Paul on the outskirts of the southern Polish city of Krakow.

He said Jesus' call to followers to minister to the world is relevant today to all in the church.

"This call is also addressed to us. How can we fail to hear its echo in the great appeal of Saint John Paul II: 'Open the doors'?" Francis said to rows of priests in white robes and nuns sitting in pews on the side.

A year after John Paul II was elected pope in 1978, he returned to his homeland, urging millions of his beleaguered Poles behind the Iron Curtain - in nuanced and coded words - to oppose communism. That visit inspired the birth of Solidarity, a labor movement that struggled through the 1980s but eventually became a key factor in the collapse of communism in 1989 in Poland, and throughout the Eastern Bloc.

At the end of the mass on Saturday, Krakow Cardinal Stanislaw Dziwisz, who for decades was John Paul's closest aide, told Francis the church remains open.

"We are not closed," the cardinal said. "We are open to the needs of the church."

The religious celebrations came on the fourth day of the pope's five-day visit to Poland, his first ever visit to Eastern Europe.

The 79-year-old Francis has had an unrelenting schedule since he arrived in Poland on Wednesday for World Youth Day, a global Catholic gathering. He has led masses, visited Auschwitz, and met with Polish politicians, clergy, sick children and many faithful.

Francis began his public day with a visit to the Divine Mercy Sanctuary, a half-mile stroll away from the St John Paul II shrine.

In 2002, a frail, 82-year-old John Paul II consecrated that new basilica during his last visit to his homeland, anointing its white marble altar. John Paul stressed then his special attachment to St Faustina, whose accounts in her diary of visions of Jesus spread devotion to Divine Mercy.

Francis prayed before the chapel of St Faustina, where she is buried.

Going into the church, the pope paused to see a young girl whose artificial legs were paid for by Francis, Vatican spokesman Father Federico Lombardi said. The Argentinian pope also heard confessions from seven young people and a priest, speaking in Italian, Spanish or French.

From there it was a quick drive to the hilltop Sanctuary of St John Paul II. That church was consecrated in 2013 and dedicated to the late pope. The lower church hosts a glass container of blood from John Paul, who died in 2005, while his body is entombed in a lower level of St Peter's Basilica at the Vatican.

Francis at lunch with 12 volunteers at the youth gathering. One of the lucky few, Paula Mora of Colombia, said "it was like being with our father, and we were his children".

Francis then rested for a bit ahead of an evening vigil with the youth in a huge meadow just outside Krakow. Pilgrims filled the meadow hours ahead of the event amid high security.

He will end his visit to Poland on Sunday after a mass in the same meadow, the crowning event of the youth jamboree.