Actress Dame Emma Thompson has told Extinction Rebellion protesters her generation had “failed young people”.
Speaking on a pink boat in London’s Oxford Circus, she said she felt “inspired to support the movement”, which has seen more than 570 arrests since Monday.
Hundreds of police officers surrounded the boat as she spoke on the fifth day of demonstrations.
The Met warned protesters it had “strong plans” in place.
Dame Emma, 60, said: “We are here in this little island of sanity and it makes me so happy to be able to join you all and to add my voice to the young people here who have inspired a whole new movement.”
She joined the protests in London after flying from Los Angeles on Thursday.
It comes as a small group of demonstrators staged a protest at Heathrow Airport amid threats to disrupt flights over Easter.
Protesters stood by the tunnel leading to Heathrow Terminals 2 and 3, but all roads remain open.
However, Robin Ellis-Cockcroft, 24, said the group had succeeded in creating an “emotional disruption” at Heathrow, adding that no further Extinction Rebellion protests would take place on Friday.
Protests are still being held at Waterloo Bridge, Oxford Circus and Parliament Square, but the Met Police said officers have been working 12-hour shifts and have had leave cancelled.
Ken Marsh, chairman of the Metropolitan Police Federation, said: “This is very, very difficult for us because my colleagues have never come across the situation that they are faced with at the moment.
“They are dealing with very, very passive people, probably quite nice people, who don’t want confrontation whatsoever with the police or anyone else but are breaking the law.”
Environmental activists have also been in action in other parts of the world.
In Paris, they blocked the entrance of the Societe Generale bank headquarters, as part of a protest urging world leaders to act on climate change.
Pepper spray was used by anti-riot police in an attempt to disperse the demonstrators.
Activists also gathered outside the Ministry of the Ecological and Inclusive Transition in La Defense, near Paris, and blocked the entrance of the headquarters of French oil giant Total there.