"At Økern, [metro station] we have found a boy, seven years old, Asiatic appearance, Mongoloid, green pullover," the police reported over their official Twitter account @oslopolitiops.
According to the Norwegian Encyclopedia, the term "Mongolism" is no longer used to refer to people with Down's Syndrome, a congenital disorder caused by an additional chromosome, because "it is perceived as discriminatory".
Arne Lien, the head of the Norwegian Association of the Disabled, told Norway's VG newspaper that the police had gone "way over the line".
"It's shocking that the public sector should represent a human being in that way," he said. "This way of describing a person reveals an attitude that's highly discriminatory."
Camilla Schreiber, Chairman of the Norwegian Down's Syndrome Network, said she believed the tweet had probably been the result of ignorance rather than prejudice.
"What I choose to believe is that this is probably a good cop who doesn't have a clue," she said. "It is shocking that today's educated police choose to use such a term when no other part of society does. It's sad if they do not know which term to use today."
The police rapidly deleted the message, replacing it with a tweet describing the boy as having Down's Syndrome.
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Ola Krokan from the Oslo police said he "strongly regretted" the message.
"I did not mean to offend anyone, and it will not happen again," he said.