The fire is believed to have broken out in the Mandarin Oriental Hotel near the Capital's West End.
The 12-storey Mandarin Oriental Hyde Park had just completed the "most extensive restoration in its 115-year history".
TV presenter Anna Whiteley a took to Twitter to announce that she had been evacuated from the hotel.
Dawson said staff and guests were evacuated within minutes to Hyde Park, and some people looked "very shaken".
The hotel recently underwent a major refurbishment.
Roads between Scotch Corner and Hyde Park Corner have been closed while emergency services deal with the blaze.
The cause of the fire was not immediately known and there were no immediate reports of injuries.
"We have not treated any patients but we remain at the scene as a precaution", Norton said.
Firefighters wearing breathing apparatus were searching the building to see if anyone still was inside the area.
Firefighters are now battling a huge blaze that has broken out at the Mandarin Oriental Hyde Park in London.
Dawson said the fire appeared at the very top of the building, where new penthouses are now being constructed and are covered in scaffolding.
The London Ambulance Service sent multiple crews to the scene of the fire, including its hazardous area response team, according to assistant director of operations Graham Norton.
Knightsbridge is shut down between Sloane Street and Grosvenor Place.
Mandarin Oriental said that "as the situation is still unfolding, we have no further details to share at this time, but will provide an update soon". The Duke and Duchess of Cambridge held a gala dinner in the grand ballroom before their wedding in 2011.