'Off with their heads!' shouted the crowd as Charles and Camilla met rage in Regent Street

It was a routine day of handshakes and small-talk for the royal couple. Then they took a wrong turn, writes Andy McSmith

It was a busy day for the Duchess of Cornwall. She had a public engagement at lunch time, and another in the evening. That was twice the number of entries in her published diary as in her husband's.

Theirs is a closeted existence, free of the stresses of normal life. Charles's vast income from the Duchy of Lancaster relieves them of the need to work for money, though they try to make a few public appearances every week in support of good causes. And though we might envy their unearned wealth, most of us would find their lifestyle excruciatingly dull. Nothing much happens to disturb the routine of royal life.

But, for a few terrifying minutes on Thursday night, the couple were confronted by a section of the population who did not want to shake their hands or listen to their inconsequential small-talk. It appeared, fleetingly, as if the rioters on Regent Street actually intended to do them harm.

They damaged the car in which the couple were travelling, giving them a nasty fright. A photographer caught their shocked, open-mouthed expressions. In another photograph, the Duchess could be seen leaning forward as if she was about to cower on the floor, while her 62-year-old husband's face went fiery red with anger and alarm.

Other people know what it is like to be intimidated by aggressive behaviour in the streets after dark, but for the heir to the throne and his wife to experience such behaviour has not happened before in modern times.

The Duchess had recently agreed to be patron of a charity for underprivileged children run by a small group of London taxi drivers. That day, the cabbies and their wives came to Clarence House for lunch, along with a group of special needs children, who were assigned the task of decorating the Clarence House Christmas tree. "What a great day!" said a notice on the website of the London Taxidrivers' Fund for Underprivileged Children.

The other half of the Duchess's working day should have been less demanding, when it was her turn to be entertained. The Royal Variety Show is one of the longest established charity events in the royal calendar, raising money for performers who have grown old or hit hard times.

Charles and Camilla were due to be driven the short distance from Clarence House to the London Palladium, near Oxford Street, to be greeted by Lord and Lady Lloyd-Webber, and sit in the VIP box to watch the show hosted by Michael McIntyre and featuring Susan Boyle, Cheryl Cole, Take That, and many more.

Having seen the television pictures of the disturbances outside Parliament, the Prince reportedly joked: "Hopefully we'll be able to brave our way through."

Actually, the couple had no reason to worry. Their route did not take them close to Parliament. They were travelling in a Rolls-Royce Phantom VI, donated to the Queen to mark her silver jubilee in 1977. The police laid on motorcycle outriders, while officers from the Special Escort Group travelled a short distance ahead of them, and passed along the entire length of Regent Street without setting eye on any protesters. But just as the royal motor entered Regent Street from the Oxford Circus end, about 300 protesters who had broken away from the main demonstration in Whitehall ran up the same street from the Piccadilly end.

It was the luxury make of the royal car and the presence of the motorcycle outriders which caught their attention. About 20 of them swarmed around the car, throwing missiles and shouting "Off with their heads!" The Rolls-Royce has reinforced windows, but the passenger window on Camilla's side was wound down, reportedly enabling a rioter to push a stick through and prod her in the ribs.

One person who had run into Regent Street with the protesters was a professional photographer, Matt Dunham from the Associated Press, who had been barred by the police cordon from returning to the main demonstration. A combination of luck and determination earned him the action shots that were on every front page yesterday morning, and an appearance on ABC's breakfast show.

The royal couple arrived at the theatre shaken but unhurt, though their car had a shattered rear window and its boot was covered in white paint. After the show, they were whisked home in a police van, like prisoners on their way to court.

Yesterday, the Prince made another public appearance, to present operational medals for service in Afghanistan to members of the Commando Helicopter Force.

Given the vast numbers of police who were present at Thursday's demonstration, there were predictable questions asked about why the police were not on hand to protect the couple – although how they were supposed to foretell that a breakaway group would turn up in Regent Street at that precise moment is not altogether clear.

A statement from the Prince's press office said: "Although we are not able to comment on any of the specifics of the incident, Their Royal Highnesses totally understand the difficulties which the police face and are always very grateful to the police for the job they do in often very challenging circumstances."

But the fact is that this is the worst show of hostility any of the royals have had to face on London's streets in living memory, other than from Irish terrorists. Now that it has happened once, it could happen again, and, as the public spending cuts deepen, Charles and his wife may wonder whether one is altogether safe behind the reinforced windows of a conspicuous Rolls-Royce.