He remained in a critical but stable condition in hospital as a man, 30, from Norfolk, was held on suspicion of attempted murder.
The suspect was on a day trip to Johnsons of Old Hurst zoo near Huntingdon, Cambs, when the incident happened around 1.30pm.
Police believe the pair did not known each other.
A local said: “It was an organised outing.
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The adult has learning difficulties and had a carer but the carer apparently wasn’t watching him at the time.
“Apparently, the wife of the zoo owner, Tracey Johnson, jumped in to the enclosure to save the child.”
Johnsons, founded by Tracey’s hubby Andy, said its “thoughts and prayers are with the boy and his family”.
It added: “Out of respect, our tropical house will remain closed until further notice. The rest of the site will remain open.”
The local added: “Apparently the boy has a broken pelvis and a broken arm, but thankfully the crocodile didn’t actually get to him.
“Someone was able to jump in and get him out of the water.
“It could have been so much worse, but he’s obviously got some really serious injuries from the fall.
“None of the injuries were from the crocodiles, thank God.”
A villager said they saw tearful staff hugging each other after the incident, adding: “It’s simply awful.”
Visitor Amelia Foster said she, her partner and son were guided to the exit, adding: “The police, ambulance service and staff handled everything exceptionally well.”
The zoo’s stars are Nile crocodiles, which can grow to 16ft.
Mike Annicelli, 52, who lives nearby, and has visited with his kids, said: “Some areas have plastic glass to look through.
“There is also 4ft fencing around the walkway so it is impossible for people to fall in accidentally.
“It felt very safe. There was a 15ft drop to the crocodiles. Staff were also keeping an eye out.”
Cambridgeshire Police Det Insp Verity McCann said: “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.
“We do not believe the man arrested and the child are known to each other. Officers are supporting the boy’s family at hospital.”
East of England Ambulance Service said an air ambulance was called to the scene but the boy was taken by road to Addenbrooke’s Hospital in Cambridge.
Local Tory MP Ben Obese-Jecty said the police investigation was being treated as a critical incident.
The zoo is set in 12.5 acres of woodland and 320 acres of permanent pasture.
It houses more than 100 animals from lions and tigers to cattle and deer. It became Britain’s first crocodile farm in 2006.
Andy fell in love with alligators on a trip to Florida and adopted several from a private collector.
The zoo has featured on ITV’s Britain’s Tiger Kings with Ross Kemp.
Crocs live in the tropical house, an old cow shed, and are primarily used to dispose of livestock.
Two large pools contain Nile crocodiles and Siamese crocodiles with names such as Cuddles, Kisses and Romeo.
There are also areas for alligators and caimans.
Andy told in 2019 how it should have been “lights out” after his head was caught in Romeo’s mouth as he tried to stop a fight.