Police have issued a safety warning over a climate protest rally in the southwestern city of Bristol on Friday which will be addressed by teenage activist Greta Thunberg.
The event has been organized by the Bristol Youth Strike 4 Climate group, part of a global movement of school students who stage protests in school time over what they say is the lack of government action on climate change.
Organizers say they expect between 15,000 and 60,000 protesters from across the country to attend.
One coach company said it was providing transport from 13 places around Britain, including Oxford, Birmingham, Brighton and Cardiff.
Police and the local council in Bristol issued a joint statement, expressing safety concerns.
"We have seen a number of protests over the last year however this one will be significantly larger," it said. "Please do not underestimate the scale of this protest."
Police will close roads around the area where Thunberg, 17, is expected to speak before she joins a march through the city.
Thunberg has been in Britain since the weekend. On Tuesday she visited the University of Oxford, where she met Malala Yousafzai, the 22-year-old Nobel Peace Prize winner and campaigner for girls' education, who is studying there.
The pair shared photos of themselves with their arms around each other. Thunberg called Yousafzai her role model, while Yousafzai said on Twitter: "She's the only friend I'd skip school for."
Thunberg starting missing lessons two years ago to protest outside the Swedish parliament building. She has since sparked a global movement calling for sustainability and awareness of climate change.