UPDATE: 2.44pm

A helicopter has been hovering over the scene in Marshfield and has just left.

UPDATE: 2.08pm

Forensics officers are at the scene and a digger is expected to begin excavating a raised bank this afternoon.

A police dog has searched the field as well as officers on foot, who have now mostly left the area.

A family of travellers living the other side of the road said they hadn't heard anything about the police raid.

UPDATE: 2.06pm

British Red Cross staff and volunteers are providing "humanitarian assistance" to an individual following a police operation in Cardiff, Monmouthshire and Newport today.

A spokesperson from the Red Cross said: “Our volunteers are trained to provide practical and emotional support to people in crisis. We are giving practical help such as food and drinks.”

UPDATE: 1.13pm

A cordon of around 400 metres in diameter was imposed around Cariad Farm Peterstone this morning, when search warrants were executed. Three people are still living at the Peterstone site, police said.

Search teams were preparing to excavate the area with a digger in search of a body that may be buried at the site. A raised bank, about 9 metres in length, will be excavated in stages.

Police officers were also searching the site more generally for evidence. They contended with mud as they combed back and forward across a field at the centre of the slavery investigation.

Moving between the boundaries of the field in a line of around 10 officers, they shared the field with six ponies.

Around ten police vans, including one with dogs, were at the scene at different times, coming and going throughout the morning.

UPDATE: 1.03pm

A woman who owns the yard neighbouring that being searched by police said she didn't know why police were investigating her neighbours, saying: "They're my friends - they're all lovely. They've got horses, I've known them for years."

She added: "there were lots of police down here this morning. My dad was here and they came in here. We don't know what's going on. The police are always down here."

UPDATE: 12.20am

Police say a man has been taken from the Peterstone address to an undisclosed medical reception centre where his physical and mental condition can be assessed and by specialist staff. The man whose age is unknown at this time is of Eastern European origin.

UPDATE: 11.30am

FOUR people have been arrested and over 100 police officers searched addresses in Peterstone, Penhow and Cardiff today as part of an investigation into slavery in Gwent.

The action comes after a 43-year-old man was found to be living in Peterstone in “very poor conditions” having been reported missing by his family 13 years ago.

Gwent Police also believe there may be a body buried at the Peterstone address. Specially trained officers, equipment and a forensic archaeologist have been deployed to investigate this.

The four arrested, a 66-year-old man and a 42-year-old both from the Peterstone area, a 36-year-old man from the Cardiff area and a 42-year-old woman from the Penhow area remain in police custody.

Detective Superintendent Paul Griffiths who is leading the investigation, which is known as Operation Imperial, said a specialist team of detectives has been investigating and gathering intelligence to ascertain whether this part of a larger criminal conspiracy.

He added: “The offences that are being investigated are extremely serious and we would urge anyone with any information which could assist the team to contact Gwent Police on 101 quoting log 66 23/09/13.

"Alternatively if someone has information and would rather not pass this directly to the Police then I would urge them to contact Crimestoppers, which is independent of the Police and guarantees complete anonymity, on 0800 555 111. They don’t need to give their name and the Police will never know who they are. Any information given would be passed to the team in complete confidence.

Gwent Police is working in conjunction with SOCA, other Forces, UK Human Trafficking Centre, RSPCA and the Red Cross in this investigation.