Pope Francis has capped a packed tour of New York by celebrating mass at Madison Square Garden (MSG), after greeting crowds in Central Park and praying for world peace at the 9/11 Memorial.

About 20,000 people packed into MSG, the home of the Knicks basketball team, to take communion in the presence of the 78-year-old pontiff.

The city's premier concert venue, originally booked Friday by Billy Joel, was lit up by the singer's own crew, who turned it into a serene and beautiful venue for Catholics to celebrate mass.

Once again, the hugely popular pope focused on society's most vulnerable during his last public remarks in the country's financial capital, a city of both extreme wealth and poverty.

He praised big cities for their diversity but called on worshippers not to forget "the faces of all those people who don't appear to belong, or are second-class citizens".

"They are the foreigners, the children who go without schooling, those deprived of medical insurance, the homeless, the forgotten elderly."

This marked his last public engagement in New York before he leaves for Philadelphia, where he is set to greet huge crowds at the Festival of Families, which takes place every three years.

Earlier on Friday, the pope led a gathering of 700 at Ground Zero, where he paid tribute to the nearly 3,000 who died and those who were first in line, responding to the September 11 attacks.

The head of the world's 1.2 billion Catholics led a multi-faith prayer for world peace bringing together Hindu, Buddhist, Sikh, Greek Orthodox, Muslim and Jewish leaders.

"In this place of pain and remembrance I am full of hope," said the Argentine pope.

"I hope our presence here sends a powerful sign of our wish to share and reaffirm the wish to be forces of reconciliation, forces of peace, of justice."

'Abuse and usury'

After an impassioned early morning speech at the UN, and the grave solemnity of Ground Zero, Francis was welcomed in song and laughter on a visit to a Catholic school in New York's East Harlem neighbourhood on Friday afternoon.

Beaming and relaxed, even submitting to a selfie or two, the pope, who is already a week into an exhaustingly packed US tour, seemed to come alive during the hour he spent with the children at Our Lady Queen of Angels school.

Flag-waving youths chanted: "Holy Father, we love you!" as the pope smiled and grasped hands, before heading inside the school to a welcome from a choir of largely black and Latino students.

From Harlem, Francis set off for Central Park, where he was greeted by a sea of 80,000 people who had waited to catch a glimpse of him.

He had begun the day at the UN General Assembly by offering his vision of a better world in a wide-ranging speech.

Francis touched on the persecution of Christians, the Iran nuclear deal, drug trafficking "silently killing millions", and the rights of girls to an education.

The first Latin American pope, who has seen his own country suffer economic crises, called on the UN Security Council and financial lenders to "limit every kind of abuse and usury".

He also gave his latest passionate plea to protect the environment, as he voiced confidence that a looming UN summit on climate change would reach "effective" agreement in Paris.

The pope also offered a strong endorsement of Iran's agreement with the United States and five other world powers to limit its nuclear programme - a deal vehemently opposed by many US politicians.

Source: AFP