Pope Francis

Orthodox believers were asked not to take part in Roman Catholic services and a Church delegation due to attend also stayed away.

But Church officials said the decision had been taken by mutual agreement.

It was one of the smallest crowds seen at an outdoor papal Mass during Francis's foreign trips.

People who did attend in the capital Tbilisi said afterwards that the papal visit was good for Georgia.

"This is a very significant event, both for the country and for faithful from the whole Catholic parish," Keti Khitarikhvili told Reuters news agency.

More than 80 people died when Mohamed Lahouaiej Bouhlel plowed a 20-ton truck into crowds on the Promenade des Anglais seafront in Nice during a fireworks celebration on France's key national holiday.

Ten children were among the victims of the ISIS-inspired attack. More than 200 people were injured.

"I wish to share in your pain, a pain that is even greater when I think of the children, or indeed entire families, whose life was taken from them suddenly and so dramatically," Francis said.

Mark Zuckerberg handed over a Facebook drone when he, and his wife Priscilla Chan, met Pope Francis on Monday.

"It was a meeting we'll never forget," Zuckerberg said. "You can feel [the Pope's] warmth and kindness, and how deeply he cares about helping people."

The women had been rescued from their pimps and are being given shelter and protection at an apartment run by a Catholic charity in Italy's capital.

The pontiff chatted to the women, some trafficked from Africa and elsewhere in Europe, for more than an hour.

The 79-year-old cleric has repeatedly described human trafficking as a "crime against humanity".

The Pope sat down with the women, including seven Nigerians, six Romanians and four Albanians, and listened to their stories of forced prostitution, the Vatican said.

Speaking onboard the papal plane as he returned to Rome from Poland, the Popetold reporters why he doesn't use the word "Islam'' when discussing terrorism.

"There are doubts. 'In dubio pro reo'," he said, using a Latin expression meaning that a defendant may not be convicted by the court when doubts about his or her guilt remain.

"We must avoid a media verdict, a verdict based on gossip," he told journalists aboard the papal plane during the return trip from Poland, where he had headlined a Catholic youth festival.

"It's in the hands of the justice system and one cannot judge before the justice system."

At the World Youth Day gathering, the pontiff urged young people to embrace hope and put prayer at the centre of their lives.

The event took on a carnival atmosphere with pilgrims dancing, singing and waving flags.

The visit marks the 1,050th anniversary of Poland's adoption of Christianity.

Some reports said as many as three million pilgrims, mostly young people, were at the mass, held in a vast field called the Campus Misericordiae near the southern city of Krakow.

Police declined to give an estimate of numbers.

He warned lounge chairs gave the illusion of safety from pain, fear or worries, allowing the sitter to kick back and lose themselves for hours in the latest television show or their smartphones.

"For many people it is easier and better to have drowsy and dull kids who confuse happiness with a sofa," he told an estimated one million people gathered at a vigil in a vast plain near Krakow," he said.

"Dear young people, we didn't come into the world to vegetate ... we came for another reason: to leave a mark.

The Pope called for clergy to leave their comfort zones and tend to the needy on the peripheries of life.

He is on a five-day visit to Poland to mark the 1,050th anniversary of the country's adoption of Christianity.

He is also taking part in World Youth Day, a global celebration for young Catholics.

The Pontiff is due to attend a youth prayer vigil later on Saturday.

Pope Francis began his day with a visit to the Divine Mercy Sanctuary, consecrated by John Paul II during his last visit to Poland in 2003.

He will walk in silence around the concentration and extermination camp where 1.1m million people, mostly Jews, were killed during World War Two.

On the third day of his visit to Poland, the pontiff will also meet camp survivors and Poles who risked their lives to hide Jews from the Nazis.

He is in the country to mark 1,050 years since it adopted Christianity.

Francis will be the third Pope to walk through the main gate of Auschwitz, under its infamous inscription "Arbeit Macht Frei" - work sets you free.