Around 120 firefighters and 20 fire engines have been called to a blaze at a five-star hotel in central London.
Images on social media showed huge plumes of smoke coming from the Mandarin Oriental in Knightsbridge.
London Fire Brigade said the cause of the flames was not yet known.
It has taken more than 35 calls about the fire, which is billowing smoke that can be seen widely across the capital.
The fire brigade said it was called at 3.55pm. Crews from Chelsea, Kensington, Hammersmith, Battersea and other surrounding fire stations are at the scene.
London Ambulance Service said it had sent a "number of resources" to the scene.
The 12-storey Mandarin Oriental describes itself on its website as "one of London's most celebrated five-star hotels" that "exudes an enticing mix of elegance and luxury".
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