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Pope Francis spoke to world leaders Friday morning, marking his second day in New York City in an international trip that first took the pontiff to Cuba, then Washington D.C. before heading to the Big Apple.

The pope, speaking to world leaders gathered for a U.N. General Assembly summit, championed the causes of protecting the environment, helping less affluent nations, aiding vulnerable people and avoiding war.

Francis expressed gratitude for the men and women of the United Nations who have worked to bring peace around the world.

“I pay homage to all those men and women whose loyalty and self-sacrifice have benefitted humanity as a whole in these past 70 years,” the pope said. “In particular, I would recall today those who gave their lives for peace and reconciliation among people…the many United Nations officials at every level who have been killed in the course of humanitarian missions, and missions of peace and reconciliation.”

The pope warned about the proliferation of nuclear arms.

“An ethics and a law based on the threat of mutual destruction – and possibly the destruction of all mankind – are self-contradictory and an affront to the entire framework of the United Nations, which would end up as ‘nations united by fear and distrust.’”

The pope has made environmental issues his central theme during his trip to the U.S. He is also expected to take up immigration during a rally in Philadelphia on Saturday.

The pontiff raised the plight of people fleeing their lands, specifically mentioning the refugees from Syria, among a few other nations.

He said that people must never be seen as mere statistics.

“There are individual persons, our brothers and sisters, men and women, young and old, boys and girls who weep, suffer and die.”

Francis' agenda on Friday reflects both his global stature and his of-the-people approach, taking him from the solemnity of ground zero to the struggles of East Harlem.

His visit was scheduled to include events as large as a processional drive through Central Park, as personal as meetings with schoolchildren and immigrants, and as inspiring for the faithful as Mass for thousands at Madison Square Garden.

Francis was greeted on his arrival at the U.N. by Secretary-General Ban Ki-moon, a key supporter of Francis' agenda. Francis thanked Ban and the U.N. staff for their service.

While his visit marks the fifth time a pope has been to the United Nations, the Vatican flag was raised for the first time just before his arrival. The General Assembly recently agreed to allow the U.N.'s two observer states, the Holy See and Palestine, to fly their flags alongside those of the 193 member states.

After the U.N., the pope was scheduled to visit the 9/11 memorial, where two waterfall pools mark the outlines of the World Trade Center's twin towers before they were toppled by the terrorist attacks of Sept. 11, 2001.

He was expected to meet relatives of some of the nearly 3,000 victims before heading belowground to the Sept. 11 museum for an interfaith service.

Francis' plans for Friday afternoon reflected the penchant of the "people's pope" for engaging with the public.

First on the agenda was a visit to Our Lady Queen of Angels School, set amid public housing in the heavily Hispanic neighborhood of East Harlem.

Known for ministering to the downtrodden in his native Buenos Aires, Francis was set to meet schoolchildren and offer a special blessing to refugees and immigrants, including people living in the country illegally.

Then he was to greet as many as 80,000 onlookers during a drive through Central Park, en route to Mass for 18,000 at Madison Square Garden.

On Thursday evening, thousands cheered as Francis waved from his popemobile along Fifth Avenue en route to St. Patrick's Cathedral for evening prayers.

His reflections included his strongest expression yet of gratitude and respect for American nuns, whom he thanked for their strength, spirit and courage.

Pews full of American priests and sisters erupted in applause on hearing Francis' words, which came after he halted an overhaul the Vatican had ordered under his predecessor to the largest umbrella group of U.S. sisters. The Vatican office that guards orthodoxy had accused the nuns of straying from church teaching, which they denied.

The Associated Press contributed to this report.

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