Christians began fleeing early June 9, Archbishop Amel Shamon Nona of Mosul told Catholic News Service in an e-mail.
"The soldiers and all policemen left the city," seemingly abandoning the people, Nona wrote. He called the situation a tragedy.
"And we started organizing, working all through the night, to find a place for them (the people who had fled Mosul) in nearby villages," the archbishop said.
News reports said several hundred heavily armed members of the al-Qaida-inspired Islamic State of Iraq and the Levant occupied government facilities, broadcast stations and banks as they worked their way through the city.
The BBC reported that up to 500,000 people left Mosul as the ISIL forces occupied the city. Many headed to three towns in Kurdistan in northern Iraq, where temporary camps were established.
The exodus from the city of 1.8 million, about 350 km north of Baghdad, continued throughout the night and day June 10, with many of the people leaving on foot with no belongings, Nona said.
"I met with a family who have two young children, two and three years old, who walked for five hours through the night, and they were very scared. We placed them in one house with another family in Telkef (in my diocese, three km from Mosul)," the archbishop said.
"All our churches in Mosul are closed, and there is not one Christian in the city," Nona added.
The archbishop also pleaded for international assistance to step in to provide aid to the people fleeing Mosul.
Iraqi Prime Minister Nouri al-Maliki called on parliament to declare a state of emergency June 10 after the government lost control of Mosul and parts of Nineveh province.
A U.S. State Department spokeswoman condemned the seizure of Mosul, calling the situation "extremely dangerous" and urging the country's fractious political groups to fight Iraq's enemies together, Reuters reported.
The Islamic State of Iraq and the Levant began as an Iraq-based affiliate of al-Qaida. It is fighting to establish an Islamic state in parts of Iraq and Syria. Al-Qaida's central leadership has denounced the organization for apostasy and excessive violence. It is estimated to have 3,000-5,000 fighters.