Bournemouth beach was evacuated as rescue workers pulled at least two people from the sea and took them to hospital.
The Coastguard pulled at least two youths from the waves, who were reportedly dragged out to sea.
Hundreds of people enjoying the warm weather were asked to leave the beach on Wednesday afternoon to allow an air ambulance to land.
South West Ambulance Service said two people were taken to hospital. His condition has not been made public.
An eyewitness told ITV Meridian that he saw three people – possibly teenagers – being rescued from the water.
Emergency services were called just after 4.30pm, including the South West Ambulance Service, two air ambulances, Poole and Southbourne Coastguard rescue teams, an RNLI lifeboat and Dorset Police.
A police cordon was put up around the area and a nearby flyover was closed for traffic.
A Dorset Police spokesman said the force received a report at 4.32pm from the ambulance service requesting assistance on the beach at Bournemouth Pier.
“Officers are currently attending to clear the area to assist HM Coastguard and the Ambulance Service, who are dealing with the incident,” he added.
Several witnesses said on social media that they saw two air ambulances overhead, as well as police jet skis searching the water.
Groups of emergency service personnel gathered on the beach, and there were unconfirmed reports that CPR was being performed by professionals.
A Coastguard spokesman’s statement read: “Two people were pulled from the water and passed into the care of an ambulance service. Coastguards carried out searches to ensure no other people were missing, and satisfied that there is none.
The Ambulance Trust said it dispatched two air ambulances, six double-crewed land ambulances, a critical care car, two operations officers, a doctor, a hazardous area response team and a response officer.
“We took two patients to the Royal Bournemouth Hospital and Poole Hospital,” he said.
University Hospitals Dorset NHS Foundation Trust says it is “working on a response” to what happened.