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."