Part of the roof of a four-storey block of flats caught alight on Griffiths Road in Wimbledon. Fire crews were first called at around 9.54am, with teams from across south and west London attending.
Three men, who were working on the building, have been taken to hospital by the London Ambulance Service.
Five fire engines attended the blaze, the London Fire Brigade said in a statement, which had been brought under control by 11.16am.
It is not currently known what caused the fire.
Footage from the scene shows the block of flats alight under scaffolding, with grey smoke pouring out from underneath a plastic canopy.
Fire chiefs said they received eleven calls about the fire due to the amount of smoke that had been produced by the scaffolding boxing in the flames.
Another eye-witness reported “extensive” road closures in the area, with police swooping to shut down the street.
Images show fire engines, including a Fire Command Unit, lining up along Merton Road.
Station Commander Dean Wilkinson, who was at the scene, said: “The fire was on the flat roof of a four-storey block of flats.
“Due to the sheeting around the scaffolding at the top of the building, the blaze produced a lot of smoke which prompted a high number of calls to our Control Room.
“Due to the sheeting it wasn’t immediately clear how much of the roof was involved in the fire.
“Once crews had accessed the roof they quickly extinguished the fire, which involved building materials, and were able to limit the damage.
“The three people taken to hospital were builders working on the roof and they were taken as a precaution due to the amount of smoke.”
Four bus routes in the area have been placed on a temporary diversion due to the building fire.
Buses have diverted via Kingston Road and Hartfield Road towards Colliers Wood or Morden, according to TfL travel updates.