Protester James Goddard has been arrested in connection with incidents outside Parliament on Monday.
Police said a man in his 30s was arrested outside St James’s Park Tube station in central London just before midday.
He is being held on suspicion of a public order offence.
BBC home affairs correspondent Daniel Sandford said Mr Goddard’s supporters said he had been about to hand himself in at a London police station.
A Met Police spokesman said Mr Goddard was currently in police custody and was being taken to a central London police station.