Hundreds of people inside the London courthouse Thursday afternoon were evacuated due to a gas leak.

Judges, lawyers, clerks and prisoners had to leave the building at 80 Dundas St. about 2:30 p.m.

Police say the source of the gas leak was outside of the building and came from adjacent construction work on Ridout Street. It entered into the courthouse ventilation system and began to circulate.

The odour was first detected in the basement prisoner holding area so the lower level was evacuated first. Prisoners were transported to the Elgin Middlesex Detention Centre.

The remaining building was evacuated shortly after that and the courthouse was closed to the public as a precautionary measure.

All business at the courthouse was postponed, but the building is expected to re-open Friday for regular business hours.