Hundreds of foreigners have allegedly encroached the Dugadda water source in the Chilla range at Rajaji National Park near Rishikesh.
The "adventure enthusiasts", who gathered here through an adventure website, have left park officials red-faced.
The administration has made repeated attempts to remove them but in vain.
The officials said more than 400 foreigners from 51 countries were staying in the park. But the administration came to know of it three days ago.
Residing inside the park area without permission is prohibited and the "unwanted guests" have not sought prior permission.
"We have informed the higher authorities," Shravan Kumar, deputy director of the park said.
"We have repeatedly requested them to cooperate with the administration but they refused to do so."
The group, accompanied by a seer from Haridwar, were seen singing bhajans (hymns), country music and strumming guitars. They had also erected camps.
Authorities believe the group entered the forest with food and water, and were getting supplies from villagers.
Wildlife warden S B Lal said, "They want to stay inside the forest till 13 April. We are planning to book them under forest acts. We have requested the (Kumbh) mela administration to arrange for police for the operation... we will move them out of the forest."