A bizarre activity called "flash mobbing" has come to Britain from the United States and several cities are set to be targeted by the craze.
Crowds of people arrive "from nowhere" during a flash mob
Since June, spontaneous crowds summoned up via the internet have been assembling in cities around the world and taking part in a form of performance art.
The first British case took place on Thursday night when 300 people converged on a London furniture shop as it was about to close, all chanted the same phrase, and then left as quickly as they had arrived.
The craze has already spread to Europe and one website dedicated to the subject has links to future events planned for UK cities.
People register to take part on the internet and are then told, by text messages or e-mail to turn up at a designated public location at a specific time and await further instructions.
Flash mob targets
A flash mobbing website explains: "The 'mob' appears without warning and dissipates as swiftly as it arrived - like a school of fish or flock of birds."
One of the first "flash mobbings" took place in New York in June when scores of people, who had first been told to gather in various bars, suddenly descended on the rug department of the famous Macy's store and surrounded one rug to apparently debate its merits.
In another incident a crowd swooped on the Mezzanine floor of a New York hotel at 1900 EST and began applauding loudly for 15 seconds before disappearing.
Sofa Appreciation Society
Derrick Robinson, manager of Sofas UK in central London, where Thursday night's "mobbing" took place, said it nearly did not happen at all.
"We normally open until 7pm but because it was hot I had closed up and had gone to the pub," he said.
"But somebody came in and said there were 250 people stood outside the shop and there were cameras as well. I thought it might be a celebrity trying to get in so I rushed back.
"They all asked to be let in and I kept saying `why?'. Then one of them said they were from the Sofa Appreciation Society, which I knew wasn't true, but I thought it was a good excuse anyway.
"They were only inside about 15 minutes when about 100 mobile phones went off.
"Then all of a sudden they were gone."