NEW YORK — Sweeping through the landmarks of America's biggest city, Pope Francis on Friday offered comfort to 9/11 victims' families at ground zero, warnings to world leaders at the United Nations and encouragement to schoolchildren in Harlem as he mixed the high and low ministry so characteristic of his papacy. In the early evening, he led a jubilant parade through Central Park past a crowd of about 80,000 and celebrated Mass at Madison Square Garden, usually the site of basketball games and rock concerts but this time the scene of a solemn service celebrating New York in all its diversity. "Living in a big city is not always easy," Francis told 18,000 people at the Garden, easily one of the most respectful crowds the arena has ever seen.