The police are increasingly cracking down on flashmobs generated by social media sites like Facebook.

Sussex police have announced that they will not allow an attempt on the world's biggest custard pie fight, despite originally giving consent.

The Pie Fight Brighton was due to have taken place on Saturday. Over 1,200 people signed up and the event had to be cancelled over fears that the crowd would become uncontrollable.

A pie fight spokesman told The Daily Telegraph: "At the time the police agreed to let it go ahead there were less than 100 people signed up. Now there's more than 1,200 and unsurprisingly the police aren't too happy about it.

"The police have a good sense of humour and think it's a fun idea and have suggested we organise the event for real. It's not as spontaneous as it was but it could still easily be a world record."

Meanwhile police in Leeds have banned a mass pillow fight organised on Facebook over similar fears.

"While such an event may seem like a bit of fun, it is easy to see how a mass pillow fight could get out of hand and potentially cause our officers a significant amount of work," said a police spokesman.

The ban came after an event in Leeds over the May Day bank holiday in which 350 people took part in a water fight organised on Facebook.

The participants caused thousands of pounds worth of damage to the Millennium Square Garden and police are examining YouTube footage to identify offenders.

However, several events are still being organised and many pass off without disruption.

New York's Grand Central Station and London's Trafalgar Square have hosted mass freezes, where participants hold still for five minutes before continuing as though nothing had happened.