Around 40 people are being treated by paramedics following reports of a chemical leak in the swimming pool of a holiday park.
Police, along with fire and ambulance crews were called at around 2.35pm to Wild Duck Holiday Park in Belton near Great Yarmouth, Norfolk.
A Norfolk Constabulary spokesman said: "It is believed there are around 40 casualties who are being treated by ambulance crews.
"The immediate area has been evacuated and a 20 metre cordon has been put in place around the building whilst emergency services deal with the incident."
Wild Duck Holiday Park is run by the holiday chain Haven and set within trees and lakes in a forest.
On its website it promises customers a "relaxing time" and the resort boasts a restaurant, play area, sports court and pools, including a seaside themed outdoor pool.
Its website states: "Our turquoise pools are a great place to go for a swim.
"Our outdoor pool is open from 22 May till the end of August - for sunny days.
"Our indoor pool is heated - great for those rainy days. Take a swim or have a splash at our at our pool party. We even have some paddling areas for toddlers."
Haven Holidays spokesman Tom Cassidy said that 33 people described as "walking wounded" had been evacuated and taken to three different hospitals as a "precaution".
A three-year-old was among the casualties taken to Norfolk and Norwich Hospital, James Paget Hospital in Great Yarmouth or Ipswich Hospital, Mr Cassidy said.
He told the Press Association: "There was a chemical leak in the swimming pool in the entertainment complex and it was evacuated while police and fire and rescue were on their way.
"Thirty-three people have been taken to three local hospitals as a precaution. They include one three-year-old child and a member of staff.
"The casualties have been described to me as walking wounded.
"Obviously the park is now in lockdown while they try and source the chemical leak."
An East of England Ambulance Service spokeswoman said: "The ambulance service was called at 2.31pm to the Wild Duck Holiday Park to reports of a chemical leak.
"A number of emergency vehicles have been sent including three ambulance officers, five ambulances, two critical care paramedics and the Hazardous Area Response Team. The first officer was on scene in five minutes.
"We are currently assessing and monitoring 29 patients at the site who are suffering with breathing problems, none of whom are thought to be serious condition.
"In addition, eight patients have been taken to the James Paget Hospital with asthma and breathing problems, all of whom were in a stable condition.
"Ambulance crews continue to work closely with both police and fire colleagues."
Mr Cassidy said ambulance crews at first treated around 40 people before removing 33 to hospital.
He said: "Team members from the resort have now gone to each hospital to liaise with the guests.
"As most of the guests were evacuated wearing only swimwear, other team members have gone to their caravans and removed clothing to take to the hospital.
"Other guests at the resort were later allowed back into their caravans although the swimming pools and entertainment complex remains sealed off while health and safety officials carry out investigations."