Manager Giacomo Maccioni said: "A police officer came in and said that everyone had to go. People were in the middle of their meals, but we had to ask them all to leave."
There were initial fears when the blaze broke out in three three-storey buildings that some of the nation's art treasures were at risk.
But Ms Stevens said she was never worried the fire would spread to the main RA building, Burlington House. "At no point was any artwork in danger. There was no artwork at all in the building that caught fire."
Ms Stevens said the cause of the fire was not yet known.
An RA security guard, who asked not to be named, said: "They were putting up frames and boards for the exhibition, then the fire alarm went off and everyone had to get out."
The fire was extinguished at around 5pm but firemen were still spraying water on the smouldering roof late yesterday evening.
Guy Foster, from the London Fire Brigade, said: "A section of the roof and the third floor level have been severely damaged by the fire. The crews have limited it to the roof and terrace of the west wing."
He said the fire was not being treated as suspicious.
While an alternative venue for USA Today is possible, it is being curated by Royal Academy Exhibitions Secretary Norman Rosenthal who is anxious that it should be the RA that hosts Saatchi's show.
This means it is likely to be postponed at the RA until winter at the earliest. The news will be a blow to the RA which had planned USA Today as the first of an annual autumn series of contemporary art shows in the Burlington Gardens space.
It is not the first time that fire has wreaked havoc on Saatchi's artistic fortunes. In May 2004 a warehouse fire in Leyton destroyed more than 100 of his pieces, including artworks by Hirst and Emin.
