A spokesperson for Air Ambulance Charity Kent Surrey Sussex said both of its helicopters and critical care team had been sent to the scene as part of the response, and Shoreham RNLI said its lifeboat had been called out.
South East Coast Ambulance Service (Secamb) said it had been called to “reports of four people in need of medical attention”.
A section of the A259 – a main commuter road which runs through Brighton, Shoreham and Worthing – was closed.
BBC reporter Callum French said despite warm weather, sea conditions had been “pretty rough” because of high winds, describing the response as “the biggest turnout [of emergency services] I have seen“.
Witnesses described seeing large numbers of emergency vehicles and helicopters arriving at the seafront.
Resident Shirley Pumfrey said: “I have never seen them do that. The whole road was just full of emergency services.
“It was horrific and quite scary to see that many.”
Steve Chapman, who lives opposite the beach, said he could not get into his road because it was cordoned off.
“There were all sorts of rumours flying around, it was very upsetting,” he said.
Another resident, Pete Book, said he had been walking home from the pub when he saw ambulances, coastguard crews and helicopters converging on the area.
Shelley Ayres, who also lives opposite the beach at Shoreham, said: “The sea has been very calm for months and months, but today it has got a lot rougher.”
Tom Rutland, East Worthing and Shoreham MP, earlier urged people to stay away from the area while emergency services dealt with the incident.
After news emerged of the deaths, he said his thoughts were with the family and friends of those who had died and he added: “I know the whole community here in Shoreham will be hoping and praying for the recovery of the young girl, external receiving medical treatment in hospital.”
Shoreham Port said it was working with multiple agencies and also urged the public to avoid the area.
