The Reclaim the Streets organisation led its supporters to a site on the M41 at Shepherd's Bush that had been kept secret from the police and most demonstrators as well.
There they barred one of Britain's shortest motorways, setting up sound- systems that pumped out high-volume rave music for road protesters to dance to on the tarmac.
The anti-roads group assembled supporters near Liverpool Street station, in the City, then led them to Notting Hill Gate station via the Central Line.
Activists had already set up 20ft-high steel tripods to block approaches to the M41 from the busy A40(M) Westway into central London.
Other organisers, wearing pink wristbands or ribbons, told the demonstrators when to leave the Tube trains and led them to the motorway. Police were unable to stop the demonstration because of the numbers involved, estimated at around 3,000.