Extinction Rebellion activists are launching legal action against the police over a London-wide ban on their protests.
The move comes amid growing criticism of the ban, made under public order legislation already used to restrict the action to Trafalgar Square.
Activists continued protests in the capital in defiance of the police order, targeting the Department for Transport and locking themselves to a caravan on Millbank, prompting more arrests.
Human rights lawyer Tobias Garnett, working for Extinction Rebellion, said the group would be filing a High Court claim challenging the ban on the grounds it is "disproportionate and unlawful".
The group was planning to file a claim on Tuesday afternoon, and was seeking an expedited hearing.