More than 100 firefighters are tackling a major blaze at the Mandarin Oriental hotel in Knightsbridge.
Twenty fire engines and 120 firefighters are at the scene of the 189-bedroom, five-red-AA-star hotel, in William Street, which had just completed a £85m renovation.
Pictures posted to social media showed huge plumes of smoke pouring from the roof of the historic 12-storey building, which overlooks Hyde Park, shortly after 4pm.
Katherine Price, The Caterer's reporter at the scene, said the amount of smoke rising from the building appeared to be lessening an hour later, as firefighters worked to prevent the blaze spreading.
A full evacuation of the hotel took place after the alarm was raised, with London Fire Brigade now carrying out a systematic search of the building.
Hotel staff were evacuated to Hyde Park and a large cordon remains in place around the building.
At 5:55pm London Ambulance Service released a statement which said: "We have not treated any patients, but we remain on scene as a precaution."
London Fire Brigade station manager Pam Oparaocha, who is at the scene, said: "The smoke from this large fire was very visible across central London this afternoon and we received a number of emergency calls. Firefighters are working extremely hard to bring the fire under control. Crews wearing breathing apparatus are carrying out a search inside the building.
"Firefighters are likely to be there throughout the evening and once the fire has been brought under control fire investigators will take over to establish how the blaze started."
Crews from Chelsea, Kensington, Hammersmith, Battersea and other surrounding fire stations attended the fire, the cause of which is unknown at this time.
Nearby hotels, including the Capital, are offering support to Mandarin Oriental guests and staff, including delivering bottles of water to those in the park.
Mandarin Oriental Hotel Group, a member of the Jardine Matheson Group, currently operates 31 hotels and eight residences in 21 countries. It has operated its London hotel since 1996, when it acquired what was then known as the Hyde Park hotel from Forte. It later closed the hotel to undergo a £57m restoration, reopening in 2000.
The group tweeted: "Thank you for your messages and for thinking of us. The situation at Mandarin Oriental Hyde Park is still unfolding. We will provide an update once we have more information to share."
The hotel's most recent renovation, completed earlier this year, was the most extensive in its 115-year history and incorporated all guest rooms, suites and public areas as well as the hotel's spa, which is understood to have reopened today.
The management team at the Mandarin Oriental is headed by general manager Gérard Sintès and executive chef Ivan Alvarez. The hotel is home to two restaurants: the two-Michelin-starred Dinner by Heston, overseen by Heston Blumenthal and Ashley Palmer-Watts, and Bar Boulud.
Palmer-Watts tweeted: "Everyone one from our team at DINNER BY HESTON is ok, the Restaurant will be closed this evening and will post an update in due course. Thank you for the messages."
Last year the hotel was the subject of a Channel 4 documentary, A Very British Hotel.
The A4 in Knightsbridge between the A3216 Sloane Street (Scotch Corner) and the A302 Grosvenor Place (Hyde Park Corner) has been closed in both directions while emergency services work.