Angry student protesters erupted in violence in the streets of London immediately after the House of Commons voted to triple university fees.
They swarmed Prince Charles and Camilla sitting in a vintage Rolls-Royce on their way to a theater Thursday.
Someone smashed one of the car's rear windows and splashed it with white paint. Some protesters chanted "Off with their heads."
"You can't blame the royal protection squad for a bunch of anarchists' bad behavior," said Alex Bomberg, a former aide to the royal family and CEO of Intelligence Protection International, Ltd. "But you can blame someone for not doing their job correctly and not understanding the situation as it was unfolding. Someone's head should bloody roll."
Photos showed Charles and Camilla visibly shaken, but unharmed.
Buckingham Palace does not comment on royal security procedures, but security experts said the prince was lucky to have surfaced unharmed.
"It wasn't potentially dangerous - it was dangerous," said security analyst Charles Shoebridge, calling the attack "one of the most serious security breaches of the past decade."
The prince's boxy car also came under fire. Experts said it lacked speed and maneuverability and had big clear windows with reinforced, but not bulletproof glass.
Security experts also warned that procedures must be drastically improved before Prince William's wedding to fiancee Kate Middleton at London's Westminster Abbey on April 29.
Police said 33 protesters were arrested. but would not say whether any of the arrests were linked to the royal attack. Scotland Yard said most of those arrested had been released on bail.
The Associated Press contributed to this report.