The Scottish Fire and Rescue Service (SFRS) was called to the "well-developed" fire at Great George Street, in the west end of Glasgow, just after 3am on Sunday.
Six fire engines were sent to the scene and crews have been working over a number of hours to extinguish the flames at the three-storey building.
An SFRS spokeswoman confirmed that a crew was still at the scene at 10.45pm - almost 20 hours after they were alerted to the fire.
There were no reports of any casualties.
A #fire has broken out at the Belmont West housing development on Great George Street in the west end of #Glasgow.
— Radio Clyde News (@RadioClydeNews) 31 March 2019
Firefighters have been tackling the blaze at a church building since the early hours of this morning. pic.twitter.com/oMKXSfppJw
It is understood the building was being converted into flats.
Police Scotland said a joint investigation involving the fire service and police is being carried out to establish the cause of the blaze.
A spokeswoman said: "Inquiries are continuing."
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