Pope Francis

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.

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.

He told young people gathered in Krakow "a merciful heart opens up to welcome refugees and migrants", a statement that puts him at odds with Poland's anti-immigrant right-wing government.

It is Pope Francis's first visit to eastern Europe, with Poland the home of the late Pope John Paul II.

On Friday he is visiting the former Nazi death camp of Auschwitz-Birkenau.

There he saw the icon known as Black Madonna, a relic that has been venerated for six centuries.

He also said the Church should ask forgiveness for the way it had treated women, for turning a blind eye to child labour, and for "blessing so many weapons" in the past.

He made the comments while speaking to reporters aboard a plane taking him back to Rome from Armenia.

In the hour-long freewheeling conversation that has become a trademark of his international travels, Pope Francis was asked if he agreed with recent comments by a German Roman Catholic cardinal that the Church should apologise to gay people.

The meeting followed a surge in migrant traffic between Libya and Italy, with more than 14,000 saved from overcrowded boats in one week.

The Italian Navy said after three consecutive days of shipwrecks, hundreds may have died.

The Pope met migrant minors as well as Italian youngsters who let off dozens of white balloons inside Vatican grounds.

In apparently off-the-cuff remarks the 79-year-old pontiff promised to examine whether women could join the clergy at the rank of deacon, 

The pledge came in a question and answer session with members of female religious orders during a meeting at the Vatican.

In the exchanges, Pope Francis said he had discussed the use of female deacons in the early centuries of the Church with experts on the subject but was not clear as to their exact role and status.