Pope Francis was headed home Monday after enjoying a rapturous reception in Cuba and the United States for his message of reconciliation, humility and care for those in need.

But he rejected the notion of being a "star", telling reporters aboard the papal plane that his job was to be "the servant of the servants of God... It's a bit different from a star!"

Throughout the nine-day trip, huge crowds lined the streets everywhere the 78-year-old pontiff went in Cuba, Washington, New York and Philadelphia.

Francis became the first pope to address the US Congress, calling on elected leaders to take responsibility for crafting a fairer economic system, confronting global warming, restricting the arms trade and abolishing the death penalty.

In New York, he led a multi-faith prayer for world peace at Ground Zero, testament to the September 11, 2001, attacks killed nearly 3,000 people, a place that he said "speaks so powerfully of the mystery of evil."

In Philadelphia, the Vatican said more than a million people poured onto the streets for his farewell mass -- more than double the number who turned out for a service led by John Paul II in 1979.

- 'Profoundly sorry' -

The rare criticism that Francis did elicit was the omission -- from his public agenda at least -- of a meeting with victims of paedophile priests, but on Sunday he met privately with five adults who were abused as children.

"God weeps," the pope told a gathering of bishops in Philadelphia afterwards, pledging to hold accountable all those responsible.

"I remain overwhelmed with shame that men entrusted with the tender care of children violated these little ones and caused grievous harm. I am profoundly sorry," Francis said.

Aboard the Rome-bound papal plane, Francis said sexual abuse by priests was "nearly a sacrilege" and "those who have hidden (abuse) are guilty including some bishops."

Around 6,400 Catholic clergy have been accused of abusing minors in the United States between 1950 and 1980, although campaigners fear that the number could be higher.

Activists welcomed what they called a long-awaited admission by Francis that the Church had covered up abuse but said only actions, not words, would make a difference.

"Francis must publicly and promptly punish bishops who are protecting predators now. And he must order bishops to do the same with their own staff and clerics who are protecting predators now," said Barbara Blaine, president of SNAP, the Survivors Network of those abused by Priests.

- 'No Catholic divorce' -

Many Americans who camped out to catch a glimpse of Francis credited him with rejuvenating a Church still reeling from the scandals and facing declining attendance records and falling numbers joining the priesthood.

His relatively reform-minded approach to social issues -- refusal to judge gays and lesbians including priests -- his humility and his love for the most vulnerable have struck a chord across the country's racial and socioeconomic divide.

But he said Monday that divorce would remain off-limits for Catholics, ahead of a synod on the family starting Sunday and following a reform that streamlined the process for couples to obtain annulments.

An annulment "is not a divorce, because marriage is indissoluble when it is a sacrament -- and the Church cannot change that: this is doctrine," Francis said.

Non-Catholics, Christians of other denominations and followers of other faiths at all his US stops were drawn to the pope's universal messages of love and teaching by example.

On an exhausting schedule, it was when meeting the disadvantaged -- immigrant schoolchildren in New York, the homeless in Washington and prisoners in Philadelphia -- that he seemed most animated.

In Philadelphia, he stopped his modest Fiat 500 to bless a disabled teenager in a wheelchair pressed up against the barrier.

At Philadelphia's largest prison, he shook hands and offered kind words to around 100 inmates.

"All of us are invited to encourage, help your rehabilitation," he said, in a country with the largest prison population in the world.

Manuel Portillo, 54, a Guatemalan immigrant who has lived in Philadelphia for 22 years, said he had never seen such enthusiasm in the city.

"I followed every step of the pope's visit, and I think he made fantastic remarks on Cuba and immigration," he said.

- Crockery and mothers-in-law -

In Cuba, Francis urged the country to continue down the path of reconciliation, fresh from helping broker dialogue between Havana and Washington, which led to the resumption of diplomatic ties.

But in America he was perhaps most constant and firm on immigration, a hugely divisive topic in the nascent 2016 election campaign.

The son of Italian immigrants, he has repeatedly reminded Americans of their immigrant roots and the values of the nation's founding fathers.

On Monday Francis turned to the migrant crisis in Europe, speaking out against building walls and barricades to keep people out.

"We must be intelligent when this migratory wave arrives, and it's true it's not easy to find solutions... Walls are never solutions, but bridges, yes."

There were moments of levity during his US tour. Defending the traditional family in an off-script speech laden with jokes at a festival of families on Saturday, his remarks were lapped up by the faithful.

"Families quarrel and sometimes plates can fly and children bring headaches, and I won't speak about mothers-in-law," he said.

"But those difficulties are overcome with love," he added.