Firefighters are tackling a large blaze at the Baitul Futuh mosque in Morden, south London.
A London Fire Brigade spokesman said: "We have 10 fire engines and 70 firefighters there. The fire is affecting the building's ground and first floors. It would be fair to say it is a large fire."
The blaze was reported just after noon though it is not yet known what caused the fire.
One man has been taken to hospital by emergency services.
A London Ambulance Service (LAS) spokesman said: "We treated a male, reported in be in his 40s, with smoke inhalation and took him to St George's Hospital as a priority."
Roads have been closed in the suburban neighbourhood surrounding the mosque and Transport for London said the A24 into Morden town centre, is shut both ways, northbound and southbound.
Reports suggest the mosque is the largest in western Europe and can accommodate up to 10,000 worshippers in three halls.
It was completed in 2003 at a cost of approximately £15m.
