UNITED NATIONS — It will be the largest gathering of presidents and prime ministers ever at the United Nations. Some of them will assemble here under one roof for the first time in years, and their job will be to wrestle with global crises that they stubbornly disagree on, including climate change, the war in Syria, and a historic exodus of people fleeing conflict and hunger.

But even before they start lecturing one another at the United Nations General Assembly this week, the leaders will come in for a heavy dose of moral flogging by an enormously popular leader known for taking swings at the global elite: Pope Francis.

This is the first time a pope will address such a large gathering of world leaders at the United Nations, according to Ban Ki-moon, the United Nations secretary general. Francis speaks in the General Assembly on Friday morning, before the official start of a global summit meeting where an ambitious set of development goals are to be adopted, including reducing economic inequality and offering good schools for all.

Lifting up the poor is a signature issue for Francis. Yet, he could give world leaders a tongue lashing for ravaging the environment, failing to protect people from mass atrocities and rejecting refugees who show up at their borders.