Police said they were working to contain a rave involving 2,500 people in Pembrokeshire.
Roads near Dale have been blocked to stop further vehicles arriving although people are being allowed to leave.
A Dyfed-Powys Police said the activity was illegal and it wanted "to ensure minimum disruption" to local people.
Sound systems were brought onto the site on Saturday with people travelling from as far afield as Devon, Hampshire and the north east of England.
Music and dancing was continuing on Sunday afternoon at the site, which is around a mile and a half from Dale village.
Party-goers told BBC Wales they planned to clear up the site before they left.
Sound systems, some with 10ft high speakers, moved onto the grassland on Saturday and have been joined by hundreds of cars and vans.
The site itself is cliff-edge on one of the most beautiful parts of Pembrokeshire coast national park.
Police set up road checks turning away cars at junctions up to five miles away and at the lane alongside Dale Fort, which is the main access to the clifftop.
BBC Radio Wales reporter Gilbert John said a police helicopter arrived over the site on Sunday afternoon and farmers were joined by council officials in checking the activity.
He said there were no attempts to move any of the party-goers or their vehicles, with locals and the police preferring to allow the event to reach a natural conclusion, expected on Bank Holiday Monday night.
Some residents were worried that the site might be left covered with litter and because there were no properly organised toilets.
But young people at the site told the BBC they were using plastic bags to prevent litter and intended to clean the site before leaving.
Supt Alun Harris, of Dyfed-Powys Police said they were working with local people and organisations to "contain the illegal activities that are taking place in Dale".
He added: "We want to reassure residents living near the event that we are doing what we can to safeguard their community.
"Our main aim is to minimise as much disruption to the local community and surrounding area as possible whilst ensuring the safety of the people at the event while we work to curtail this illegal gathering.
"We are monitoring the situation closely and where necessary will take appropriate and proportionate action."
He said the force was also stopping and searching vehicles travelling through the force area to get to the event and have arrested a number of people for various offences.
"We know that Dale and the surrounding area of Pembrokeshire is very popular destination for visitors however we do not welcome events of this nature," said Supt Harris.
Police said anyone with information or concerns is asked to call police on 101.