In his first speech given overseas since his election in May to lead the 1.4 billion-member Church, the first US pontiff said "ambitions and choices that trample on justice and peace" were destabilising the world.
He told political leaders in Turkey that the world was experiencing "a heightened level of conflict on the global level, fuelled by prevailing strategies of economic and military power".
"We must in no way give in to this," he pleaded at an event with President Tayyip Erdogan after they held a private meeting.
"The future of humanity is at stake."
Speaking before the Pope, Erdogan said that he welcomed the pontiff's "astute stance" on the Palestinian issue, and hoped the visit would be beneficial for humanity at a time of tension and uncertainty.
In September, Leo met Israeli President Isaac Herzog at the Vatican and raised the "tragic situation" in the Gaza Strip with him.
Leo chose mainly Muslim Turkey as his first overseas destination to mark the 1700th anniversary of a landmark early Church council there that produced the Nicene Creed, still used by most of the world's Christians.
Leo, 70, landed in the capital Ankara shortly after midday to begin a crowded three-day itinerary in Turkey before heading on to Lebanon.
It will be closely watched as he makes his first speeches overseas and visits sensitive cultural sites.
Speaking to journalists aboard the papal flight from Rome, Leo said he wanted to use his first overseas trip to urge peace for the world, and to encourage people of different backgrounds to live together in harmony.
"We hope to ... announce, transmit, proclaim how important peace is throughout the world," the Pope said at the beginning of the three-hour flight.
"And to invite all people to come together, to search for greater unity, greater harmony."
