Hollywood star and BAFTA winner Joaquin Phoenix led an animal protest during his stay in London and urged people to "go vegan".
Phoenix joined vegan activists at Tower Bridge as they dropped a massive banner from the landmark which read: "Factory farming destroys our planet. Go vegan."
The actor attended the demonstration just hours before Sunday's BAFTA Awards ceremony, where he received the prize for the best lead actor for his role in Joker.
Phoenix, who is an animal rights activist, has been a vegan since he was three years old.
Speaking to Sky News, he said: "People are not in a panic in the way they should be. These are very dire times, and so I felt like I had to do something.
"We have a personal responsibility to take action right now.
"One way we can mitigate climate change is by adjusting our consumption and by going plant-based. I feel sometimes that's not being talked about enough.
"This is an emergency. We have to do everything we can to try to cut down our impact on climate change."
Phoenix also commented on criticism that people in the Hollywood film industry receive for preaching about climate change while taking frequent jet flights and leaving high carbon footprints.
He said: "We are all hypocrites in some ways. We all struggle with what the right thing to do is and we make mistakes.
"The industry does consume a lot of power and a lot of resources so the way to mitigate that for me is to maintain a vegan lifestyle."
Phoenix convinced the Hollywood Foreign Press Association, to serve vegan menu at the Golden Globes award ceremony's last month.
He said: "I think that is part of our appeal to the award ceremonies because there are so many people flying in to the awards and the cars they use - the one thing they can do is to make the meal plant-based.
"I think we just have to try and find that balance. Nobody is perfect but we are all trying to figure it out.
"Instead of being judgmental and attacking each other, we should try to help each other and educate each other about how we can change.
"There are many improvements I can make in my own life and I am trying to do that."
Phoenix appeared at the protest in support of Animal Equality, which is an international organisation working to end cruelty to farmed animals.