An unknown armed group set fire to the building of an Egyptian Coptic church on Thursday in Benghazi, Libya's second largest city, according to witnesses.

The witnesses said the militants fired shots in the air at first and then set fire to the church on al-Mehdwi Street in the downtown area of the eastern Libyan city.

The fire, which produced much flame and smoke, caused no casualties, but burned the two-storey building to the ground, witnesses added.

The incident came after the Libyan authorities said this week that 55 Egyptian Christians arrested last month in Libya for allegedly seeking to convert Muslims had been freed, while four other foreigners arrested in mid-February for proselytizing were still behind bars, and one of them, Ezzat Hakim Atallah, an Egyptian Copt, was dead.

A Libyan security official said on March 1 that these Copts were arrested on illegal immigration charges, adding they were found printing Bibles and texts encouraging conversion to Christianity, something strictly prohibited in Libya.

The Egyptian Coptic Youth Front claimed that Atallah was tortured to death.

His death sparked a demonstration on Monday in Cairo, Egypt where dozens of Christians threw stones at the Libyan embassy, burned a Libyan flag and called for an immediate investigation into the incident.

They also demanded an official apology from the Libyan government.

The attack on the Coptic church was likely a response to the protest in Cairo.

Coptic Christians make up some 10 percent of Egypt's 83 million people and constitute the largest Christian group in the Middle East.

Libya had as many as 100,000 Christians before the 2011's uprising that toppled long-time leader Muammar Gaddafi, but only a few thousand now remain, according to church officials.