Ravers are thought to have travelled from across the UK to an illegal event in Wales attended by up to 3,000 people, despite new rules to crack down on mass gatherings.
Officers are at the scene in Banwen, Neath Port Talbot, and South Wales Police have condemned the "totally unacceptable" event.
It comes as police in Norfolk worked to shut down an all-night unlicensed music event in a forest attended by more than 500 people.
New rules which came into force on Friday ahead of the bank holiday weekend mean organisers of illegal raves could be hit with £10,000 fines.
Chief Superintendent Simon Belcher of South Wales Police said the force is "looking at all pieces of legislation" as they deal with the huge crowds.
He said "This type of illegal gathering is totally unacceptable and we are aware of the concerns it is causing for the local community.
"I would like to again remind people of their obligations under the current coronavirus legislation and the overarching goal for everyone to take personal responsibility by following Welsh Government regulations to Keep Wales Safe.
"We have a number of resources in attendance including the police helicopter and roads policing officers.
"It has been estimated there are up to 3,000 people at the gathering who have apparently travelled from across the UK and we are looking at all pieces of legislation as to what action can be undertaken safely."
He said unlawfully parked cars "will be dealt with" and people who turn up to the event will be turned away.
Norfolk Police described the organisation of a large gathering in Thetford forest, which began late on Saturday night and which officers believe was due to take place throughout the long weekend, as "extremely disappointing".
The force said they would have to deploy extra resources to close the event down.
Assistant Chief Constable Nick Davison, who is leading the Norfolk Police operation, said a drone unit, dogs, the National Police Air Service helicopter and specially trained public order officers had all been deployed to an isolated area of Thetford forest, near to the English Heritage site Grimes Graves.
He said it was clear the "well-established and planned event" was due to continue into Monday and the force was working to "close the event down in a controlled manner, taking the safety of those in attendance and our officers into account".
He said: "It's extremely disappointing to see these types of events taking place when the rest of our communities are doing everything they can to help stop the spread of the virus."
West Yorkshire Police said eight people were fined a total of £10,000 after several parties across Headingley and Burley on Saturday and into the early hours of Sunday.
The force also said one man has been charged with causing a public nuisance after officers broke up a street party on Wepener Mount, Harehills, on Saturday. Five other people at the party have been fined.
Meanwhile, thousands of pounds worth of equipment has also been seized in the police crackdown.
West Yorkshire Police seized what they believe to be around £20,000 of music equipment from a garden at an address in the Chapeltown area of Leeds on Sunday which officers suspect was being kept for an unlicensed music event.
It came after Essex Police said it had also seized thousands of pounds' worth of equipment – but did not give an exact figure – ahead of an unlicensed music event the force said was due to take place in Harlow on Saturday afternoon.
West Midlands Police said it had dealt with about 90 reports from the public about possible breaches of restrictions by Sunday morning "but we've not had to use our enforcement powers".
National vice chairman of the Police Federation of England and Wales Che Donald said the legislation should be clearer to allow policing to be "more effective".
He said: "Our police officers are doing an incredibly difficult job in testing circumstances but as always, they will be ready to deal with anything that may come their way over the long bank holiday weekend."
Metropolitan Police Federation chairman Ken Marsh also called for clarity around the new legislation which he said would allow officers to "be more forceful clearing the area immediately, close the area down".
Andy Rhodes, Chief Constable of Lancashire Constabulary, argued confusion over the guidelines is being used by some as an "excuse" to break the rules.
Home Secretary Priti Patel has defended the new rules, which came into force on Friday, saying it represents a crackdown on "the most serious breaches of social distancing restrictions".
Writing in The Daily Telegraph, she said: "We will not allow this breathtakingly selfish behaviour from a senseless minority to jeopardise the progress we have made together."
The Metropolitan Police force alone has responded to more than 1,000 unlicensed events since the end of June.