The NO2ID group said they were arrested on Thursday morning on the grounds they may cause a breach of the peace.
The campaigners, who were going to wear orange boiler suits and bar codes on their foreheads, had created a massive ID card to highlight what they see as an increasing restriction on civil liberties.
A spokesman for NO2ID said:
"It is shocking that we could not have a peaceful protest in a peaceful country but this is what these ministers want."
A spokeswoman for the police said:
"Four men and two women have all been arrested on suspicion of conspiracy to commit criminal damage and are currently in custody."
Update: Spy Blog is not impressed.