Nine people were evacuated from a building which is alight in central London, as more than 70 firefighters and 10 fire engines tackle the blaze.
London Fire Brigade were called to the incident at a house in Wimpole Street in Marylebone at 1.27am.
Three people were checked for smoke inhalation, London Ambulance Service said, but a spokeswoman for London Fire Brigade confirmed no-one was hurt.
The Metropolitan Police said they were called to the scene at 2am.
The fire service said three storeys of the terraced house were on fire.
They tweeted: " More than 70 firefighters are dealing with the WimpoleStreet fire in Marylebone. Nine people were led to safety from the building."
Chris Hawkswell of London Ambulance Service tweeted: "Residents have been evacuated from the building, 3 people have been assessed for smoke inhalation, hospital not required."
Pictures from the scene showed the top part of the building ablaze with fire crews using specialist equipment to tackle the flames.
A person tweeting under the name Josef said: "Fire on Wimpole street looks pretty serious - roof alight on 5-storey terrace, hoping everyone is out of the building."
London Fire Brigade Station Manager Jim Morford, who is at the scene, said: "When they arrived firefighters were confronted with a large fire in a terraced town house and crews have been working hard to stop it from spreading to neighbouring properties.
"Nine people have been led to safety and the buildings on either side of the affected house have been evacuated as a precaution.
The fire service said the blaze is now affecting four of the building's five storeys as well as the roof.
Two turntable ladders are being used to help fight the blaze from above, it added.
The cause of the fire is unknown and the service said crews are likely to remain at the scene "well into this morning".