Pope Francis implored bishops not to cover up the sexual abuse of children, during an open-air mass on Sunday that capped a three-day visit to Belgium, which is still struggling with the legacy of past scandals.

Gathered in bright morning sunshine, about 40,000 faithful, some waving Belgian and Vatican flags, packed the King Baudouin stadium north of the capital to hear the 87-year-old pontiff.

"There is no place for abuse, there is no place for covering up abuse," Francis said during a homily.

"I ask everyone not to cover up abuse, I ask the bishops not to cover up abuse, to condemn the abusers and help them heal themselves of this disease of abuse."

During his stay, the pontiff was pressed by Belgian authorities and victims' advocates on the Catholic Church's handling of child sexual abuse, which was put back on the front pages by a hard-hitting documentary last year.

The Belgian documentary prompted around 200 more people to come forward with allegations that they were abused by members of the Church, adding to around 1,000 cases reported before then.

On Friday, Francis met with 17 victims. "I felt their suffering," he told the Sunday mass.

"Evil must not be hidden, evil must be brought out into the open, let it be known," he added, calling for all perpetrators to be judged.

Women and values

In a sign of the work yet to be done, the program of Sunday's mass had to be changed at the last minute after it emerged that the closing hymn was composed by a priest accused of sexual abuse.

The blunder prompted the head of the Belgian bishops' conference, Archbishop Luc Terlinden, to admit that the Church needed to better monitor cases and perpetrators. 

While the question of abuse cast a shadow throughout his stay, the Argentine pontiff was given a rapturous welcome at Sunday's mass, where young people and families cheered as he toured the venue in his Popemobile.

"He transmits all kinds of values," Olivier Caillet, 44, from Brussels, said of the pope, praising his welcoming stance on migration.

"He's a bit like the pope of surprises, he's with the underprivileged... he's opening up new possibilities, trying to change people's consciences so that we don't close borders like barriers."

Belgium's King Philippe and Queen Mathilde were also in attendance at the mass, which Francis used to appeal for an immediate ceasefire in Lebanon and the wider Middle East.

"I call on all parties to immediately cease fire in Lebanon, Gaza, the rest of Palestine and Israel," the pontiff said, as Israel continued to target the Hezbollah armed group in Lebanon.

Francis's visit, the first by a pope to Belgium since John Paul II in 1995, also highlighted the growing disconnect between the Vatican and some faithful in progressive, secular countries over issues such as gender inequality.

On Saturday, he was challenged during a frank exchange with students about the place of women in the Church — after paying tribute to a former sovereign who took a public stance against abortion.

His response, describing women as daughters, sisters and mothers upset some, with the Catholic university hosting the meeting later expressing its "incomprehension and disapproval" at the reductive view.

"I don't understand why women can't become priests. It's something that was established because it was rooted in society at the time, but now we've moved on," Alice Vanwijnsberghe, an 18-year-old student, told AFP after attending a festival with 6,000 young Catholics.

The pope left Brussels at around 1:30 PM (1130 GMT), taking off from the Melsbroek military airport outside the city.

He was expected to give his traditional press conference on board the plane taking him back to Rome.