Police outriders alerted a mob to a royal car by honking their horns and riding "aggressively" towards them, an eyewitness said yesterday.
A convoy carrying Prince Charles and the Duchess of Cornwall was "well behind" the group of student tuition fee protesters who surrounded the vehicles, it was claimed.
But when motorcycle outriders tried to push their way through on their way to the London Palladium, demonstrators turned on the distinctive Rolls-Royce Phantom VI.
The Duchess was poked in the ribs with a stick as a window of the car was smashed, bodywork kicked and white paint thrown over it.
The eyewitness came forward after an internal Scotland Yard review of the incident was submitted to Home Secretary Theresa May.
He revealed the Duchess appeared to wind down her window in the chaotic scenes in Regent Street in the West End.
The eyewitness said the confrontation emerged after several police outriders "aggressively" pushed ahead into the noisy crowd.
He added: "The demonstrators scattered, but then some of them realised who was in the car so they surrounded it.
"One of the car windows was then wound down, which is when someone poked a stick through it."
The Royal couple were travelling to the Royal Variety Performance when they were caught up in violence surrounding the vote on tuition fees.
The Duchess was pictured looking shocked and distressed and they were forced to leave afterwards in the back of a police van.
Metropolitan Police Commissioner Sir Paul Stephenson ordered an internal review, the key points of which were presented to Mrs May on Friday.
But Assistant Commissioner John Yates, who controls the specialist operations wing of the force, also asked for more detail and was handed a revised version yesterday. The review is expected to call for increased security around members of the Royal Family.