A woman was 'violently mugged' in Bromley town centre last Thursday, while around 40 bystanders stared and did nothing.
Little did they know the incident was part of an experiment set up by makers of a Channel 5 documentary, and the victim and mugger were both actors.
They acted out the bag snatching in Bromley High Street to see how members of the public would react when faced with a street mugging.
But while around 40 people witnessed the crime, only one man rushed to intervene.
Detective Chief Inspector Andy Furphy of Bromley police said: "We were aware in advance that this 'experiment' was being filmed and taking place in Bromley.
"Fortunately this type of incident is extremely rare in Bromley town centre and it's probably because of this that it was such a shock to those who witnessed it taking place.
"While the actions of the man who intervened were highly commendable, we wouldn't normally recommend members of the public intervene in this way if they're witness to a genuine robbery.
"The best thing to do is to call 999 immediately, taking note of the suspect's description - clothing, height, hair colour or any distinguishing features and wait for police to arrive."
The experiment featured on last night's Law and Disorder: Catching Criminals Live, 10pm on Channel 5.