Speaking at an open-air Mass before a huge crowd, he urged the international community and Arab countries in particular to find a solution to end the conflict in neighboring Syria.
"Why so much horror? Why so many dead," Benedict said, lamenting that "the first victims are women and children."
With pilgrims from across the Middle East in the crowd he said Christians must do their part to end the "grim trail of death and destruction" in the region.
"I appeal to you all to be peacemakers," Benedict said.
Benedict spoke from an altar built on land reclaimed with debris from Lebanon's 1975-1990 civil war, pressing ahead with his call for peace and reconciliation between Christians and Muslims.
The Rev. Federico Lombardi, a Vatican spokesman, said local organizers put the crowd at some 350,000 people.
Benedict said that justice and peace are needed in building "a fraternal society, for building fellowship."
The 85-year pope, wearing green vestments, appeared to be holding up well in the Mediterranean heat.
Helicopters flew overhead and soldiers set up roadblocks and patrolled streets in downtown Beirut.
POPE's remaining schedule for today:
Sunday, Sept. 16 (Beirut, Harissa, Charfet)
-- 10 a.m. Mass and presentation of the post-synodal apostolic exhortation for the Middle East at Beirut City Center Waterfront. Homily by pope. Recitation of the Angelus. Remarks by pope.
-- 1:20 p.m. Lunch with the papal entourage at the nuncio's residence in Harissa.
-- 5:15 p.m. Ecumenical meeting at the residence of the Syriac Catholic patriarch in Charfet.
-- 6:30 p.m. Farewell ceremony at Rafiq Hariri International Airport in Beirut. Speech by pope ( via newser.com ).