Eight fire engines pumped water to tackle the fire at the 28- floor building which overlooks Hyde Park.
Television pictures showed roads in the busy area of the city were closed off to traffic.
Metropolitan Police said nobody had been injured in the blaze.
An fire brigade spokesman said: "It is between the ducting on the second floor and the basement.
"We haven't received a stop message, which means we haven't been told the fire is out." As well as the engines, an aerial platform was used as a "cherry picker" along with other vehicles to survey the fire, the spokesman said.
Senior fire officers and fire investigation officers were among the 50 or so crew members battling to put out the blaze.
Earlier in the day the hotel hosted the Silver Clef Awards in which Sir Paul McCartney was honoured as the year's best live act.
Sky News said stars who had attended the event such as Annie Lennox, Liza Minnelli, Status Quo and Tinie Tempah were among the evacuees.
The annual charity event is formally known as the Nordoff Robbins O2 Silver Clef Awards. - (PA)
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Written by Irish Times