A Circle Line Party is a planned sponteneity similar to a flash mob but with fun and revelry as its main purpose. Circle Line parties are parties held on the moving trains of the London Underground train system's Circle Line. Though the time and place of these parties is pre-planned the even itself occurs spontaneously, as each participant decides for themselves what sort of revelry, costume, decorations, snacks, libations and guests to bring to the party.

These parties are intentionally unpermitted hijackings -- or reclamations, depending on your point of view -- of a public space. The trains are decorated festively by the party-goers, and music -- from pre-recorded music played on cunningly devised sound systems, designed to appear as normal luggage -- to live music performedby bands is played. Sometimes pole-dancing platformsare set up on the trains, for the 'go-go dancers' among the revelers.

Though these parties are 'sprung' on unsuspecting commuters, the commuters are not excluded, but invited to participate. Free drinks, snacks and sweets are consumed, often mysteriously appearing from well-stocked portable bars and snack tables, that disappear as the train arrives at the station.

To maintain a"low profile" -- at least whilst the trains are in the stations, most of the partygoers only revel and frolic once the trains are in tunnels. Though Circle Line parties are illegal, and sometimes trains are stopped for long periods by the police, in an attempt to re-assert control over the public space of the Underground, Circle Line parties are not meant to disrupt travel, but to "reclaim the public space from the advertisers and give it back to the people to whom it belongs" according to the Space Hijackers, a semisecret group of self-styled "anarchitects" who maintain a growing network of "Secret" headquarters in Europe and North America.

The Space Hijackers themselves are a varied lot, ranging from activists, artists, performers, performance artists, guerrilla theater players, programmers, electronic hobbyists, clothing designers... in other words, just about anyone who thinks that the public commons are becoming commercial domain, and who wish to reverse the trend.

Other Space Hijackers events have included unauthorised cricket matches in downtown London, commercial education excursions (wherein a group travels from shop to shop discussing the tricks retailers use to sell more, cheaper made, more expensive goods by using retailing tricks, such as: point-of-sale displays; placing the most desirable items in the middle of aisles, to require the shopper to pass other items to get their desired item (and perhaps a few impulse items); the use of different flooring textures to subtly guide shoppers through the stores, and similar retail ploys.

Though the space Hijackers may have publicised the first Circle Line party, the idea has become a new meme and appears to be spreading to other cities.