West Midlands Fire and Rescue Service said the "collapse was caused by ongoing structural works inside the premises" in Walford Road, Birmingham.
Fire crews were called by police at 11.25pm yesterday, and arrived to find much of the terrace property's roof had fallen in. Pictures from the scene showed a skip and safety fencing at the front of the mosque, appearing to show building work had been going on before the collapse.
More images showed the inside of the mosque piled up with roof debris. A specialist dog team from Leicestershire Fire and Rescue Service and a police helicopter were called in to search for anyone who might have been trapped in the rubble.
It appears no one was inside at the time of the collapse and nobody else was hurt, despite the mosque's location on a residential road.
A fire service spokesman said: "Structural engineers have been in attendance and contractors came out to make the building safe at about 3am.
"A K9 team has been through the property internally and externally and we're confident no persons were inside." A West Midlands Police spokesman said the incident was not a police matter but Walford Road remains closed and access may also be restricted on surrounding streets, Osborn Road, Medlicott Road and Holder Road. The force added the council had been notified and a safety inspection of the building would take place.