By Wiktor Szary, Wojciech and Zurawski

KRAKOW, Poland (Reuters) - Pope Francis will start his first trip to Poland on Wednesday to preside at an international Catholic youth jamboree, a usually joyous event that has been dampened by the murder of a Catholic priest in France.

News that suspected Islamist militants interrupted a church service in Saint-Etienne-du-Rouvray, near Rouen, forced 85-year-old parish priest Father Jacques Hamel to his knees and slit his throat, sent a sudden deep chill over the World Youth Day event.

"We listened to the news with disbelief, we got phone calls from our parents who tried to reassure us," said Guillaume from Bordeaux, a pilgrim in his early 20s.

"Pope John Paul II said that we should be tolerant and not resort to force. My heart tells me that this is true, but after what happened my head is now telling me something else," said the young man, who declined to give his last name.

Pope Francis was horrified by the killing, the Vatican said, calling it particularly "barbarous" because it happened during the central rite of Christian worship.

It shocked many Catholics because it was eerily reminiscent of the murder of Salvadoran Archbishop Oscar Romero, who was shot by a right-wing death squad in 1980 while he was saying Mass in a hospital chapel in San Salvador.

Some events at the gathering of Catholic youth from the around the world, which has been dubbed the "Catholic Woodstock," are expected to draw more than a million people and Polish authorities have laid on tight security.

A police spokesman said the current threat level had not been raised after the killing of the priest.

Early in the trip, Francis is expected to pay tribute to the murdered priest, who some have said should be declared a martyr because of the way he was killed for his faith.

Francis' five-day trip to the Krakow area of southern Poland will take place in the shadow of his predecessor, John Paul, who has cult-like status in Poland for his role in inspiring the nation to stand up to communist rule.

That status was evidenced by the fact that there are more posters of John Paul in Krakow than of Francis, an Argentine.

John Paul, who died in 2005 and visited Poland nine times as pope, was made a saint in 2014. His reign of nearly three decades was the most historic of modern times.

Poland has undergone a significant shift in its political landscape since the Eurosceptic, conservative Law and Justice (PiS) party in October ended nearly a decade of secular-minded government.

Its leaders tapped Poland's brand of patriotism infused with Catholic piety to build up popularity among voters and ensure the backing of the clergy, influential across provincial Poland.

Francis will also visit the Nazi death camp at Auschwitz-Birkenau, and pray at Czestochowa, site of the country's holiest shrine.

(Additional reporting and writing by Philip Pullella; Editing by James Dalgleish)