Thirty-nine people, including a teenager, have been found dead inside a lorry container at an industrial park in Essex.
Police were called by the ambulance service after the discovery at Waterglade Industrial Park in Grays at 1.40am.
All 39 people were pronounced dead at the scene and the lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.
The vehicle is believed to have come from Bulgaria and entered the UK at Holyhead in north Wales on Saturday.
Essex Police Chief Superintendent Andrew Mariner said: "This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process.
"We believe the lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate. We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue."
Police are appealing for anyone with information to get in touch with them, and have put a cordon in place and access to and from the industrial park is closed.
Mr Mariner added: "I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible. We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally."
Deputy Chief Constable Pippa Mills told reporters outside Grays police station that the incident was an "absolute tragedy and a very sad day for Essex Police and the community".
National Crime Agency officers have been deployed to assist Essex Police with the investigation, and three Red Cross emergency support vehicles were also seen heading into the site.
Sky News understands that police in Northern Ireland are also assisting with the investigation, with it thought that the lorry could have come into the UK from Dublin.
The East of England Ambulance Service said it had sent five ambulances, hazardous area response teams and a car from the Essex and Herts Air Ambulance "in the early hours of the morning".
"Unfortunately 39 people had died prior to our arrival," a spokesman said.
Ms Mills added that it would be a "lengthy process" to identify the bodies, and apologised for the disruption the investigation would cause businesses at the industrial park.
The industrial park is made up of courier and logistics companies, and is a short drive from the Dartford Crossing, which links Essex with Kent.
Paige Wade was driving past on the way home from work at around 4.15am and saw police tape cordoning off an entrance to the site in Eastern Avenue.
She said: "I knew it was serious because of how many police cars and ambulances were there, but the police had parked their cars across the whole access of the road so you couldn't see anything.
"There's always lorries around there as they park up there for the night."
Glen Freeland said staff at his nearby GSF Car Parts shop were unable to get into work on Wednesday morning because of the police cordon.
He added: "All we know is they found the bodies in the back of a lorry.
"It was a bit of a shock but we're not 100% sure of what's happened."
How did the lorry get into the UK?
The journey from Bulgaria to the UK via Holyhead has been described as an "unorthodox route" and it is thought the lorry would have come in from Dublin.
Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association (FTA), said: "There is a direct route to Holyhead from Dublin.
"If the lorry came from Bulgaria, getting into Britain via Holyhead is an unorthodox route.
"People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.
"It's a long way around and it'll add an extra day to the journey."
Irish premier Leo Varadkar said any necessary investigations would be undertaken if it was established the lorry had passed through Ireland.
He added: "It is a real terrible and human tragedy, and we will carry out any investigations that are necessary if it's established that the truck did pass through Ireland."
The Bulgarian foreign affairs ministry has said it had "no information" about the incident prior to UK media reports, but confirmed the lorry was registered in Bulgaria by a company owned by an Irish citizen.
It told Bulgarian newspaper Dnevnik: "Our embassy is in contact with local authorities and is reviewing the case."
Prime Minister Boris Johnson tweeted: "I'm appalled by this tragic incident in Essex. I am receiving regular updates from the Home Office & will work closely with Essex Police as we establish exactly what has happened.
"My thoughts are with all those who lost their lives & their loved ones."
He later opened PMQs by describing the incident as an "unimaginable tragedy", and told MPs those involved in people trafficking should be "hunted down and brought to justice".
Home Secretary Priti Patel said she was "shocked and saddened" by the news.
Ms Patel is MP for Witham, which is in Essex.
She tweeted: "Shocked & saddened by this utterly tragic incident in Grays. Essex Police has arrested an individual and we must give them the space to conduct their investigations."
In a further statement, the home secretary said police were working with Home Office immigration officials "to establish how this horrific event came to happen".
The home secretary later gave a statement in the Commons, describing the incident as a "tragic loss of life".
She told shadow home secretary Diane Abbott the UK would continue to work with EU and international partners to combat people smuggling.
Jackie Doyle-Price, the local Conservative MP for Thurrock, tweeted: "Sickening news of 39 people found dead in a container in Grays. People trafficking is a vile and dangerous business.
"This is a big investigation for Essex Police. Let's hope they bring these murderers to justice."
The Road Haulauge Association has said it is "shocked" by the discovery, which echoes an incident in 2000 when the bodies of 58 Chinese illegal immigrants were found in a sealed container at Dover port.
An inquest heard how the 58 immigrants banged frantically on the inside as their air supply ran out.
The Dutch driver of the lorry, Perry Wacker, was jailed for 14 years in April 2001 for the manslaughter of the immigrants, who had paid a criminal gang thousands to be smuggled into the UK.
Seven other men were later jailed by a Dutch court for their role in the operation.
Anyone with information is asked to contact Essex Police on 101 or at essex.police.uk.
Essex Police have also set up a casualty bureau for people if they are concerned about relatives in relation to the incident - the numbers are 08000560944 for UK callers and 02071580010 for international callers.