Extinction Rebellion activists are planning to cause disruption on the London Underground during Thursday's rush hour.
British Transport Police (BTP) are stepping up patrols after the activists announced plans to target the capital's Tube network, as part of their protests demanding action on climate change.
A spokesman for the group told the Evening Standard that the disruption will occur on Thursday morning as commuters make their way to work.
"We are planning to meet people and lead them to one of the sites," the spokesman told the newspaper.
BTP said protest organizers at Extinction Rebellion – or XR – had "made it clear that they intend to target the London Underground network".
Senior officers had engaged with members of the protest group "in order to persuade them in to stopping this highly disruptive and potentially criminal action," the force said.
"Despite this engagement, we are led to believe that direct action on the London Underground is still being considered," it added.
The force said it will increase the number of officers on patrol throughout the London Underground on Thursday, as well as the DLR and other rail networks.
Assistant Chief Constable Sean O'Callaghan said he was "really disappointed" that XR activists planned to target the London Underground network.
"The Tube and rail networks are one of the greenest transport methods in London, any action goes against what they campaign for and will only cause misery for London's commuters," he said.
"In addition to this, we've made it clear to Extinction Rebellion that disrupting Tube services could create a dangerous scenario where some trains are stuck within tunnels with hundreds of passengers on board.
"Likewise, trespassing or obstructing Tube services could create a serious hazard to protesters."
Mr O'Callaghan said any protesters who illegally disrupt the Tube network can expect to be arrested and could face imprisonment.
It comes as XR's bid to defy the blanket protest ban on its protests across the whole of London will come before the High Court on Thursday.
Those bringing the court action on behalf of the group, including Caroline Lucas MP and Guardian columnist and activist George Monbiot, are seeking to overturn the order imposed by the Metropolitan Police.
Lawyers representing the climate protesters will argue in a preliminary session that a full hearing of the case should happen "as a matter of urgency".
They will argue that the section 14 order imposed by the Met effectively banning all XR protests in London is unlawful.
Meanwhile, an open letter backing Extinction Rebellion has been signed by more than 100 celebrities, including actor Jude Law, and Radiohead singer Thom Yorke, who say that journalists who have called them "hypocrites" are "right".