Emergency services have evacuated a beach in Sussex after a suspected chemical leak.
People on the beach at Birling Gap reported painful, stinging eyes and breathing difficulties after a "mist" appeared.
Sussex Police said there was an "unknown haze coming in from the sea" affecting the East Sussex coastline.
Surrounding roads have been closed and people are being advised to avoid the area.
A police spokesman said the first report came in just before 17:00 BST where "up to 50 people had been affected by irritation to eyes and throats".
Officers said investigations were under way to find out the cause of the "haze".
The spokesman dismissed initial reports there had been a fire at Birling Gap, saying he did not know where they had come from.
It was definitely coming from the coast, he said - and previous incidents had involved stuff coming from industrial units in France.
Emergency services are treating people at Birling Gap and clearing the surrounding area.
People are being warned to stay away from the beaches from Birling Gap to Eastbourne and to keep doors and windows closed if living on the nearby coast.
Those caught in the mist say it felt like "being in a swimming pool with large quantities of chlorine".
Roy Page was on the beach when it happened and he told BBC South East Today: "It was like a mist coming down the beach from the west. Over a period of about 30-35 seconds I started to itch my eyes and so did other people.
"Within about two or three minutes my wife really started to itch badly because she has had some operations and they [her eyes] were stinging.
"Suddenly everyone around started getting up and either putting sea water on their eyes or dropping water on their eyes to clear it out."
The RNLI said a significant number of people on cliff tops had complaining of skin irritation, sore eyes and vomiting.
Concerns that people could be trapped on the beach saw the RNLI launch lifeboats, including one with a doctor on board.
A spokesman said: "We have been checking along the shoreline to try and ensure that everyone is safe."
He said a "plume" had drifted across the area bringing "some sort of substance" with it.
East Sussex Fire & Rescue urged people to "stay indoors with windows and doors shut and where possible, move away from the area".
It added: "If you feel any effects, such as stinging eyes, the South East Coast Ambulance Service is advising to wash with copious amounts of water.
"If you have any serious concerns then you should seek medical advice.
"People are urged not to go to hospital unless an absolute emergency."
It comes as large queues of people were seen outside Eastbourne hospital, where staff were wearing protective clothing.
Emergency services all working together on this - please listen to this advice. And leave beaches if we ask you to. https://t.co/OItoGKG7C7— Maritime&Coastguard (@MCA_media) August 27, 2017
The National Trust, which owns the land and has a cafe and visitor centre there, said it was aware of the incident and was working with the emergency services.
People on Twitter have reported the mist smelled of chlorine.
@BBC_HaveYourSay in Eastbourne town, eyes feel like I've just got out of a swimming pool, and my asthmatic brother struggling to breath.— steele. (@steele_music) August 27, 2017
Nice day at Birling Gap beach turned into a nightmare.. some sort of chlorine gas leak has made my eyes red raw :(— rebecca (@rebecca_2804) August 27, 2017