The newly appointed leader of Iraq's largest Christian denomination -- the Chaldean Catholic Church -- called for unity in this country threatened by sectarian strife.
Delly, elected by 22 Chaldean bishops at the Vatican on December 3, replaces the former patriarch, Raphael Bidawid, who died in July.
Iraq's estimated 700,000 Chaldeans constitute the largest Christian denomination in this country of about 26 million. Delly also represents Chaldeans overseas.
Wearing a white cassock with a red sash on the back, he called for brotherhood among Chaldeans, other Christians, Iraq's Shiite Muslim majority and the Sunni Muslim minority which formed the elite under the ousted regime of Saddam Hussein.
"We are just one family, the Iraqi family," Delly said before a church filled with about 300 people including dozens of nuns, members of Iraq's interim governing council and Shiite and Sunni leaders.
He pledged to work together with Muslims "in all conditions."
Monsignor Ishlemon Wardouni, in a homily to welcome Delly, said the new patriarch had been chosen amid "cruel conditions" in his country.
"We are asking God to help him in his ship, especially in these times, to reach the harbour of peace," said Wardouni, the interim patriarch after Bidawid's death.
He urged Delly to promote religious unity and to help build a new, democratic Iraq respectful of human rights.
"We should always be together, all religions, all nations together," said Wardouni.
Born in the northern province of Nineveh, Delly has a doctorate in theology and speaks six languages.
With incense and prayers, the ceremony lasted almost three hours and was conducted in both Arabic and Aramaic, the language of Christ which the Chaldeans still use in their liturgy.
It began with a procession of chanting clerics who led the new patriarch into the church. An old woman reached out from her pew to touch the bespectacled, white-bearded man as he passed.
Parishioners greeted his inauguration with loud applause and traditional Arabic ululation.
Iraqi police watched from neighbouring rooftops and searched parishioners entering the church.
Insurgents have launched armed attacks and bombings against the US-led coalition administering Iraq, and its "collaborators."
The patriarch's installation comes at a time of uncertainty for some Christians like Nasreen Thomas, 30, a dentist.
Speaking with AFP before the service, the stylishly-dressed woman expressed concern she might be forced to wear Islamic garb.
"We are waiting for the new government to decide what type of system we will be under," she said.