Pope Francis heads to Poland on Wednesday for an international Catholic youth festival to preach charity and openness to migrants, a mission made tougher by a jihadist murder of a priest in France.

The Argentine pontiff is flying in to meet youngsters from across the world at a week-long faith extravaganza dubbed "the Catholic Woodstock", but is expected to tackle Poland's rightwing government over refugee rights before he joins the festivities.

The brutal killing of an elderly priest during mass in France on Tuesday, in an attack claimed by the Islamic State group, cast a shadow over the gathering of the hundreds of thousands of pilgrims gathered in Krakow and increased concerns over security.

A string of recent terror attacks targeting civilians in Europe appears to have dampened turnout however. Around 200,000 pilgrims attended the opening mass on Tuesday, security sources told AFP, while organisers had originally expected around half a million.

The assassination of the French priest has also complicated Francis' desire to champion migrants and emboldening Polish Prime Minister Beata Szydlo and her rightwing government who have refused to take in refugees for security reasons.

In an apparent swipe at the anti-migrant US Republican presidential candidate Donald Trump, Francis also sent a video message Wednesday to Hispanic faithful in Brownsville, Texas, on the Mexican border.

"Always look to the future, don't let anyone throw up walls in your lives," he said.

Francis, 79, will meet Polish President Andrzej Duda in Krakow's Wawel Castle, before retiring for a closed door question-and-answer session with the country's bishops.

The pope, who voiced "pain and horror" at the "barbaric killing" of the priest, is likely to slam religious violence and the persecution of Christians while warning Europe not to succumb to xenophobia.

Poland is on high security alert, deploying over 40,000 personnel for the visit. Authorities also charged an Iraqi man Monday with possessing trace amounts of explosive material.

True to character, Francis is refusing to bow to security concerns, and will take to the road in his open-top pope-mobile for some stages of his trip.

- 'All mourning' -

"World Youth Day is a great celebration and we hope the attack in France will not ruin it," said Marcin Przeciszewski, head of Catholic Information Agency KAI, as groups of faithful gathered Tuesday to pray for the fallen French priest.

One 20-year old French pilgrim, who gave her name as Elisabeth, said: "I think we are all mourning to some extent, it's inevitable.

"This has to be the WYD (World Youth Day) of hope".

A group of French pilgrims gathered in Krakow for prayer on Tuesday night.

"The best answer to violence is love, peace and prayer," said one of the young pilgrims, Pierre Darme, as some of his friends prayed with their eyes closed.

The pope will likely have to work overtime to win hearts and minds in the homeland of Polish pope John Paul II.

The charismatic saint, hailed for his role in toppling Communism, sponsored conservative Catholic movements -- a legacy which sits uncomfortably with the Argentine pontiff's attempts to nurture a more flexible, compassionate Church.

"Polish Catholics probably aren't going to be welcoming the pope they really want, but given their current social and political situation, they may be getting exactly the one they need," Vatican expert John Allen wrote on the Cruxnow.com website.

- 'Testing time' -

As Europe struggles to cope with the biggest wave of asylum-seekers since World War II, Francis has repeatedly called for the protection of the downtrodden and persecuted, seeking to set an example by sheltering Syrian Muslim families in Rome.

But Poland has refused to take part in an EU deal to share the burden of migrants arriving in Italy and Greece by boat.

"Francis is expected to face a testing time," wrote Christopher Lamb in Catholic weekly newspaper The Tablet.

Many of Poland's bishops are "at odds with the direction of his papacy," particularly Francis' push to open church doors to traditional "sinners" such as single mothers and divorced people who have remarried.

The off-the-record meeting with church leaders will give the pontiff a chance to call on dissident bishops to reconsider their attitudes.

At the heart of the visit will be a meeting with Holocaust survivors at the former Nazi death camp Auschwitz, where Francis will pray for the camp's 1.1 million mostly Jewish victims, before the five-day trip winds up with the customary papal vigil and mass.

Source: AFP