A man is in an induced coma in Wellington Hospital after a fall during his arrest in Levin this afternoon.
Police said they attempted to arrest the man after 3pm today following calls from the public raising concerns about his behaviour.
"Initial information indicates that during the arrest process both the officer and the man fell to the ground, with the man hitting his head," police said in a statement.
"The officer called for medical assistance for the man. The man is now in Wellington Hospital where he remains in an induced coma."
Acting central district commander inspector Belinda Dewar said it had been a distressing incident for all involved.
"Our thoughts are with this man and his family. We are mindful that an investigation into the circumstances is in the very early stages and as such we are not able to provide any further detail at this time."
The area where the incident occurred has been cordoned off and a critical incident investigation is underway.
The Independent Police Conduct Authority would be notified, police said.