A publicity stunt for a new speedboat ride became the subject of a real-life police alert after a passer-by saw lookalikes of famous actors brandishing guns and raised the alarm.
The drama happened on the Thames at Westminster Pier as lookalikes of Angelina Jolie, Daniel Craig, Tom Cruise and Arnold Schwarzenegger helped launch the Thamesjet ride, marketed as "London's most extreme river experience".
A member of the public called the police, fearing that an armed gang was preparing to commit a crime.
Officers soon found out that the guns were fakes and the only shooting was by photographers.
Paul Ellerby, commercial director of City Cruises, which runs the speedboat rides, said of the incident: "It was a bit alarming to see four policemen jump out of a boat and run towards us - we thought maybe the pier was being evacuated for some reason.
"It soon became clear what it was about, and I think the police could see the funny side of it when they realised what was going on.
"Because we were only doing photography, not filming, we didn't have to get permission from the Port of London Authority for what we were doing - but, in hindsight, given the nature of what we were doing, it would probably have been wise."
Scotland Yard said police were called by a concerned member of the public who had seen a number of people on a boat on the Thames dressed in military clothing and carrying what appeared to be imitation firearms.
"Officers from the Marine Policing Unit attended and quickly established that these were actors taking part in a promotional event," a spokeswoman said.
"The actors and the event organisers were given words of advice and the incident was resolved amicably."
There were no arrests, she added.