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".
Mayor Sadiq Khan - who oversees the Metropolitan Police, said he had asked officers to find a way for those who wanted to protest to be able to do so legally and peacefully.
He said he had "received assurances that Extinction Rebellion are not banned from protesting in our city", adding:
Under the current order, any assembly - classed as a gathering of two or more people - linked to the Extinction Rebellion 'Autumn Uprising' in London is unlawful.
Lawyers have questioned the legality of ban, aimed at halting further protests after more than a week of disruption by the environmental activists in London, while a number of politicians expressed outrage over the move.
Responding to the ban, teenage climate activist Greta Thunberg tweeted: