Two Americans were arrested at a Japanese zoo after a man climbed into the enclosure of a baby monkey that became an internet sensation.

The monkey, a macaque named Punch, went viral earlier this year for clutching an orangutan plushie having been rejected by his mother.

On Sunday, a man dressed in a cartoonish blue suit and over sized yellow emoji head scaled the 1.5m fence at Punch’s enclosure at Ichikawa City Zoo outside Tokyo in an apparent stunt.

Footage shared on social media showed the intruder entering the animal’s area in front of visitors, while carrying a doll of his outfit and trying to keep his costume’s head from falling off. A second man filmed the bizarre encounter as it unfolded, at around 11am.

Neither man got close to the animals and were soon detained by zoo staff, a police spokesman told AFP. Around 60 monkeys, including Punch, were inside the enclosure at the time.

The Ichikawa Police Department identified the suspects as 24-year-old student Reid Jahnai Dayson and Neal Jabahri Duan, 27, who said he was a singer.

“We apologize for the concern caused by yesterday's intrusion into the monkey enclosure,” the zoo said in a statement on X. “No abnormalities were observed among the animals.

“We filed a police report with the Ichikawa Police Station last night. From tomorrow onwards, we will continue to prioritize the safety and health of the animals and take appropriate measures, while strictly addressing any rule violations.”

The baby macaque - named after the celebrated manga artist and creator of Lupin III, Monkey Punch - won a legion of fans onlineafter he was seen hugging the orangutan toy given to him as a substitute for his mother when other monkeys shooed the baby away.

Punch became so popular after footage and pictures of his encounters with the toy were shared on social media that the zoo had to set rules to deal with an increase in visitors.

Attendees were told to stay quiet and to limit viewing to 10 minutes to reduce stress for Punch and the dozens of other monkeys in the enclosure.

Zookeepers nursed Punch and gave him the toy to train him to cling, an ability newborn macaques need in order to survive.

But while he initially slept with the money every night, in the months since the incident Punch has started to outgrow the toy by socialising with adult monkeys.

“Helping Punch learn the rules of monkey society and being accepted as a member is our most important task,” said Kosuke Kano, a 24-year-old zookeeper.