Two children have died after several people - all thought to be kids - were pulled from the sea in a huge "multi-agency operation" on a busy beach.
A 17-year-old boy and a girl aged 12 were dashed to hospital after air ambulances scrambled to the "major incident" on Bournemouth beach, but medics were unable to save them.
It followed concerns a group of children, as young as 12, were dragged out to sea by a riptide.
The beach was evacuated so emergency services could attend the plight.
Witnesses reported seeing three young men being carried out to sea in the incident just after 4.30pm. One witness, visiting the attraction this half term, said at least one patient received CPR on the sand.
Missing radioactive capsule found after huge search - and it's the size of a peaAnother witness said "six children" got into difficulty in sea. There was concern therefore three other children were missing in the water, but the Coastguard service says it is "satisfied" there are no further victims.
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Dorset Constabulary confirmed, at around 10.30pm on Wednesday, the youngsters had died and that a man in his 40s has been arrested on suspicion of their manslaughter.
A spokesman for the force initially said on Wednesday: "Dorset Police received a report at 4.32pm on Wednesday 31 May 2023 from the ambulance service requesting assistance on the beach off Bournemouth Pier.
"Officers are currently in attendance clearing the area to assist HM Coastguard and the ambulance service, who are dealing with the incident."
Vehicles from multiple emergency services - including air ambulances - have been spotted at the scene.
South Western Ambulance Service confirmed two people were taken to hospital following the "sea incident".
One witness, on a day out with her family, said paramedics treated one person on the sand, and then another victim around 10 minutes later.
Steve Larcombe, who works on Bournemouth Pier for a boat trip company, said: "There were six children involved, the police said four were walking and two cardiac arrests.
"I just hope the children are okay."
Boy, 3, dies after being left in sweltering car in 34C heat 'throughout the day'Mr Larcombe said he wasn't sure if the youngsters had been jumping off the Victorian pier or were swimming under it at the time.
And beachgoer Ritta Saruchera said her daughter and her friend were in the sea at the time when they saw what happened.
She said: "They were coming back out of the water when they saw a teenage boy struggling. There was another one floating in the water.
"We went to get help from the lifeguard and inform them of what was happening. There were three young men out there and they have been searching for a fourth."
Nicola Holton was on the beach with her husband at the time. She said the sea became dangerous very quickly with "multiple" people needing help.
She added: "One lifeguard went to rescue two swimmers struggling but he couldn't bring them in. He spotted another person struggling and a second lifeguard went out to them but there were multiple people in trouble.
"The remaining lifeguard was trying to get everyone out of the water.
"The ambulance service came to the struggling person rescued by the second lifeguard.
"More lifeguards arrived to rescue another swimmer near the pier.
"My husband spotted another swimmer struggling and dragged across parallel to the beach. He ran to the lifeguards. The guard immediately went in and they picked him up on a jet ski. I will never ever get the image out of my head of him being brought out.
"They cleared the beach for air ambulance. Then another was spotted and again brought to shore to be worked on. Absolutely horrible.
"Loads of idiots ignoring lifeguard requests to get out of the water and clear the beach. People were running towards those having CPR filming on their phones."
Paul Moyce, 61, said he believed those involved had been jumping into the sea off the pier.
He said: "I think they went off the end of the pier and went out too far. They must have got caught by the current. I lived here for 61 years and I've never seen anything like it."
A local resident posted on social media: "My daughter is there. She rang me to say they were doing CPR on someone from the water, she thinks there are more."
Another witness, a mother who was visiting with her baby, saw emergency services started to arrive.
Writing on Twitter at around 6pm, one man posted: "At least one person receiving CPR"
The woman, called Eemen, heard the lifeguards running across the beach telling people to clear the area because the air ambulance needed to land.
She said lifeguards were searching the sea and there were also lifeguards on Jetski-type vessels.
Eemen, from Southampton, Hampshire, said lifeguards blocked off an section of the beach with windbreaks and chairs.
She was told by emergency services that it is a "major incident".
Dorset and Wiltshire Fire Service said: "We were called to East Beach, Bournemouth at 4.39pm to support a multi-agency incident.
"We have crews in attendance from Westbourne and Springbourne, together with a technical rescue team from Poole."
A spokesperson for the South Western Ambulance NHS Foundation Trust (SWASFT) said: “We were called at 16:07 on Wednesday 31 May to a sea incident in Bournemouth. We sent x2 air ambulances, x6 double-crewed land ambulances, x1 critical care car, x2 operations officers, x1 doctor, x1 hazardous area response team and x1 responding officer.
“We conveyed two patients to Royal Bournemouth Hospital and Poole Hospital.”
Cordons remained in place until around 6.45pm, by which time the air ambulances and patients had left the scene.
A spokeswoman for HM Coastguard said: "HM Coastguard has been assisting South West Ambulance Service after a call was received from Poole Bay RNLI Lifeguards about people in difficulty in the water.
"Coastguard helicopter from Lee-on-Solent, two air ambulances, Poole and Southbourne coastguard rescue teams, Mudeford RNLI lifeboat, Dorset Police were all sent in support.
"Two people had been pulled from the water and passed into the care of the ambulance service. Coastguards searched to make sure there were no other people missing and are satisfied there are not."