Besides being a major security embarrassment for BAA, yesterday's protest at Stansted was significant for two reasons: air passengers are now a legitimate target for protesters; and growing numbers of people are willing to risk arrest to hamper airport expansion.

Plane Stupid has evolved from a small group of radical activists into a disciplined platoon capable of infiltrating Britain's third-largest airport and shutting it down with nearly 60 volunteers.

Formed in 2005 by a loosely connected network of radical environmentalists, the group has crafted increasingly bold publicity stunts including the rooftop protest on the Houses of Parliament in February.

It has no central leadership and encourages people to make acts of protest in its name. But occasionally, Plane Stupid also organises major protests like the one at Stansted yesterday.

Familiar faces at those demonstrations include long-term activists such as Leo Murray, 32, and newer, younger recruits such as Olivia Chessell, 20. Mr Murray, a grandson of the late Lord Greenwood – a Wilson cabinet minister responsible for the first Bills regulating air and water pollution – organises protesters' annual Climate Camps.

Ms Chessell, meanwhile, is typical of the new generation of young, middle-class activists clamouring to be involved in direct action. Born in East Dulwich, London, she eschewed university to concentrate on youth work and protest.

Both were arrested yesterday and also took part in the Parliament rooftop protest. According to Plane Stupid organisers, the Stansted protest was predominantly made up of young first-timers, suggesting the group has footholds in university campuses.

The group's lack of hierarchy has not made it easier to infiltrate: last year Plane Stupid uncovered a mole from a "risk management" firm working for opponents within the aviation industry. Plane Stupid publicly exposed the apparent informer and said members had been feeding him false information for months.