A criminal investigation has been launched after 25 children were hospitalized with chlorine poisoning at an aquapark in St. Petersburg, a local prosecutor spokesman said Thursday.

The spokesman said the children were admitted to local hospitals after visiting the park, adding their condition was described as "moderate to satisfactory." It is believed that the swimming pools had been over-chlorinated.

The spokesman said a case has been launched for a breach of sanitary regulations leading to mass poisoning cases, and for failure to meet safety standards. The first offence is punishable by a three-year prison term.

The emergencies ministry said around 75 visitors to the Waterville aquapark in Russia's second-largest city started experiencing health problems, including skin complaints, on Wednesday evening.

However, Waterville officials said in a statement that only one person had sought medical help for sore eyes caused by an allergic reaction to the chlorine, which was considered normal, adding that "water tests carried out every three hours had shown no breaches of regulations."

On Wednesday 2,270 people visited the waterpark. The center has now been temporarily closed and water samples have been taken.

The worst disaster to hit a Russian aquapark was in 2004, when the roof of the Transvaal aquapark in Moscow collapsed, killing 28 people and injuring over 100.