Police are trying to disperse around 3,000 people who have gathered for an illegal rave in a forest.
Gwent Police are working with the fire service and the Forestry Commission to try to break up the crowds at Wentwood Forest near Caldicot, Monmouthshire.
A force spokeswoman said officers were alerted to the "illegal gathering" at around 1030 BST on Saturday.
Officers were sent out overnight to stop and speak to people at the scene and take details of their vehicles.
Specialist officers and the force helicopter are at the scene.
Gwent police decided it was unsafe to enforce action under the Criminal Justice and Public Order Acts which allows police to force people to move on.
The fire service is also attending a fire on scrubland in the forest which began on Sunday afternoon.
Soft option impimented AGAIN!!!!!!
Five people have been arrested after police in riot gear broke up a three day illegal rave in an ancient woodland in Monmouthshire.
Gwent Police drafted in extra help to disperse the estimated 3,000 people and around 1,000 vehicles at the illegal gathering in Wentwood Forest.
Officers seized 10 large trucks containing powerful sound equipment.
Police were alerted on Saturday night by local residents who heard music coming from the forest.
Amanda Shufflebotham, who lives near the forest in Llanfair Discoed, said villagers knew "something serious was going on".
"I didn't know if it was a police exercise at first but they (police) came thundering through the village up towards Wentwood.
"It went through the village - it was an illegal rave".
Upwards of 250 police officers were involved in the operation and according to Gwent Police it was the biggest illegal rave in the force's area.
Officers were drafted in from Avon and Somerset, Gloucestershire, West Mercia and South Wales Police to help.
Supt Jon Burley from Gwent Police said they had used "positive policing" to deal with the situation.
"There will be no reoccurrence of the rave in Wentwood over the Easter weekend so they (residents) can enjoy the rest of their Easter," he said.
Specialist officers were sent out on Saturday night to stop and speak to people at the scene and take details of their vehicles after it was decided it was unsafe, because of the numbers involved, to enforce legal action against those taking part in the rave.
The fire service, who were called in to tackle a blaze on scrubland in the forest on Sunday, and the Forestry Commission also assisted.
Doug Ellerby from the commission said: "We had thousands of vehicles in the forest at Wentwood and we're expecting a huge tidy-up operation with litter and debris, there's been a fire.
"We will have to assess the damage and it's going to be a busy week for the Forestry Commission down at Wentwood."