Hundreds of people were forced to leave restaurants and flats after a blaze broke out at County Hall on the South Bank.
Footage taken from the London Eye showed flames and thick smoke billowing up from the building, after the fire reportedly erupted in a rooftop garden.
The fire brigade said two people, a man and woman, fled before they arrived and were uninjured.
Four engines and 21 firefighters were sent to tackle the flames at about 4.45pm.
The County Hall complex houses luxury flats and an array of restaurants.
Mark Evans, director of the Locale restaurant, based in a different block of County Hall, said: "You could see flames coming off the roof, it was fairly intense up there.
"The restaurants and flats in the west block were evacuated. There were about 200 to 300 people standing around in the square."
The whole of the roof garden was destroyed before the flames were brought under control just after 5.30pm, London Fire Brigade said.
A spokesman said: "Four fire engines and 21 firefighters and officers dealt with a fire in the roof terrace of a block of flats on London’s Southbank this afternoon.
"Firefighters from Soho, Dowgate and Euston fire stations attended the scene. The cause of the fire is under investigation."
