Demonstrators have been camping near Drax power station for days to campaign against climate change.
Demonstrators organised a day of "mass action" at the camp near Selby in North Yorkshire in a bid to disrupt operations.
Police were drafted in from seven forces and prepared themselves for 2,000 protesters, but about 600 turned up.
There were a small number of minor scuffles and 38 people were arrested, police said.
There was a carnival atmosphere amongst many protesters, who dressed up for the occasion.
Police set up road blocks to prevent campaigners gaining access to the plant.
Police kept a close eye on demonstrators as they marched around the site chanting and waving banners.
Drax power station is the biggest coal-fired power station in Europe and campaigners say it should be shut down.