An "illegal rave" which started late on Boxing Day is set to be broken up by police.
About 100 cars are thought to be at the rave at Bellever, Dartmoor, which was initially reported at 23:00 GMT.
No arrests have been made, but police said a teenager had been taken to hospital after a "drug-related incident" about 1.2 miles (2 km) from the rave site.
A resident said the woods were "a mess" but there was "a good atmosphere".
Officers said a 15-year-old boy from Plymouth had been found "unresponsive" near Postbridge and was airlifted to Derriford Hospital at about 09:55 on Thursday.
He is thought to be in a stable condition.
A force spokesman said: "Our concerns are both for the people in attendance, and the impact on the local community.
"We will be making sure that this dispersal is done calmly and people are only allowed to leave the area when they are fit to do so."
Stephen Earp, from nearby Postbridge, said people had "come from all over the country".
"They're all in a great party mood," Mr Earp added.
However, he said there was concern from locals over "the mess they leave behind" at the beauty spot.