Pope Francis, leading a multi-faith prayer service at the site of the Sept. 11, 2001 World Trade Center attacks, on Friday called on people to embrace diversity and resist attempts to impose uniformity.

Earlier in a speech to the United Nations, he condemned the persecution of religious minorities and the destruction of cultural heritage, a clear reference to the actions of Islamic State militants in Syria.

"Here grief is palpable." Francis said at the reflecting pools marking the footprints of the Twin Towers destroyed in the attacks and where 2,606 people died after Islamist militants of al Qaeda crashed hijacked jetliners into the buildings. Overall, 2,977 victims died and the 19 hijackers of four planes they commandeered.

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Standing in front of the concrete "slurry wall" that held back the Hudson River from the towers' foundations and flanked by a dozen religious leaders from the Jewish, Muslim, Hindu, Sikh, Buddhist and Greek Orthodox traditions, Francis spoke to a crowd of about 700 people in an underground gallery.

"In opposing every attempt to create a rigid uniformity, we can and must build unity on the basis of our diversity of languages, cultures and religions, and lift our voices against everything which would stand in the way of such unity," said the Argentina-born Francis, the first Latin American pope, flanked by religious figures in their traditional dress or vestments.

Family members of six people killed in the attacks, including five who worked in the towers and a flight attendant who had been on one of the hijacked aircraft, met the pope near the pool that marks the former base of the south tower. The pope briefly stopped to pray at the fountain, leaving a flower before he moved on.

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Elected officials including New York Governor Andrew Cuomo, Mayor Bill de Blasio, former Mayors Michael Bloomberg and Rudolph Giuliani, who was in office a the time of the attack, and U.S. Senator Charles Schumer attended the event.

After his remarks, Francis viewed relics of the 9/11 Memorial Museum. They included a cross formed of fused steel girders found in the towers' smoldering wreckage and a copy of the New Testament found in the debris that is displayed open to the Gospel passage in which Jesus urges believers to "turn the other cheek" rather than strike back.