Police and transport chiefs were tonight drawing up plans to foil an attempt by climate change activists to target the London Underground.
Tube bosses fear thousands of commuters could have their journeys disrupted tomorrow.
London mayor Sadiq Khan today said he was 'extremely concerned' the action by Extinction Rebellion would risk the safety of Londoners.
The group's website threatened to 'nonviolently disrupt Tube services'. It follows a series of demonstrations which blocked roads in London yesterday.
City Hall said it was working with transport chiefs and the police to minimise the effect of any protests.
Extinction Rebellion has demanded a meeting with the Government.