A boy has been taken to hospital with serious injuries after he ended up in a crocodile enclosure, while a man has been arrested on suspicion of attempted murder.
Officers were called to Johnson's of Old Hurst, Huntingdon, at 1.24pm to reports of a "distressing" incident involving a three-year-old child.
The toddler has been taken to Addenbrooke's Hospital with serious injuries - and is in "critical but stable" condition, according to Cambridgeshire Police.
A 30-year-old man, from Norfolk, has been arrested on suspicion of attempted murder, police added.
Detectives from the force's Major Crime Unit are investigating.
"At this stage, we are speaking to people who were at the zoo at the time of this distressing incident to understand more about the circumstances," said Detective Inspector Verity McCann.
"We do not believe the man arrested and the child are known to each other.
"Officers are supporting the boy's family at hospital, and our thoughts remain with them."
The zoo at Johnson's is home to more than 100 animals, including crocodiles, Bengal tigers, African lions and sloth bears, according to its website.
A blog post from the zoo states that the crocodiles were initially kept to help dispose of waste meat from the butchery, but they went on to be the start of a zoo.
The crocodiles are kept in a converted cattle barn, which has metal-fenced elevated walkways looking down on large pools of water surrounded by tropical vegetation.
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Ben Obese-Jecty, MP for Huntingdon, said: "I am aware of the incident at Johnson's of Old Hurst, and have been liaising with senior officers, who are treating this as a critical incident.
"This is now a live criminal investigation, and I would ask people to refrain from speculation online.
"The police will provide an update with further information in due course.
"My thoughts are with the young victim and his family during a hugely traumatic and difficult time."
A spokesperson for Cambridgeshire County Council said: "Our thoughts are with the victim and their family, but, as this is a live police investigation, we cannot comment any further."
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