Around 40 people, including a three-year-old child, were treated by paramedics after a chemical leak in a swimming pool of a holiday park.
The pool was busy with families enjoying an afternoon dip when swimmers complaining of itchy eyes and breathing problems raised the alarm.
Guests dressed only in their swimming costumes were evacuated and the resort went into "lockdown" after the incident, believed to have been caused by a chlorine overload.
Paramedics treated holidaymakers for breathing problems, although no-one is thought to be in a serious condition after the incident at around 2.30pm at Wild Duck Holiday Park in Belton near Great Yarmouth, Norfolk.
Tom Cassidy, a spokesman for Haven Holidays, which runs the resort, said 40 people were originally treated by paramedics and 33 "walking wounded" were 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 said: "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."
He said the pool would have been busy with families and children.
He said: "It was the afternoon, it would have been peak time for families to be in the pool. There were obviously children involved."
Swimmers complained of "itchy eyes and coughing and then the lifeguards realised and they decided to evacuate", Mr Cassidy said.
Staff went to the hospitals with guests to make sure they were OK, and some returned back to the resort in the evening.
Mr Cassidy 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."
Investigators are probing what lay behind the incident, but sources said they believe it was caused by an overload of chlorine.
Police, fire and ambulance crews rushed to the resort, which is nestled in the woods.
A spokeswoman for the East of England Ambulance Service said: "We assessed 38 patients in total at the site, a mixture of children and adults.
"None of the patients are thought to be in a serious condition, but were suffering with breathing problems. 29 patients have been taken to the James Paget Hospital and Ipswich Hospital for further monitoring."
A Norfolk Constabulary spokesman said: "It is believed there are around 40 casualties who are being treated by ambulance crews."
On its website the resort promises customers a "relaxing time" and boasts a restaurant, play area, sports court and pools, including a seaside themed outdoor pool.
It states: "Our turquoise pools are a great place to go for a swim."