A police officer was bashed from behind while making an arrest at what police have called a "large gathering of anti-social road users" just outside Palmerston North last night.
Officers were called at around 1am to Works Rd at Longburn, with somebody having been hit by a vehicle.
They were met by a "volatile situation".
While one police officer was arresting a person for disorderly behaviour, he was struck on the head from behind and knocked to the ground, Detective Sergeant David Wilson said.
The officer was taken to hospital where he is being treated for concussion.
"The person who assaulted the officer has not been identified and we would like to speak to anyone who may have witnessed the incident," Wilson said.
Another person at the gathering was stabbed and is in hospital in a serious condition.
Police arrested a 19-year-old man at the time. He has been charged with wounding with intent to cause grievous bodily harm and is due to appear at Palmerston North District Court on Monday.
"If you witnessed either the assault on the police officer or the stabbing incident, or have any information that could help police with their enquiries, please call 105 and quote file number 220212/2718," a police statement says.
Information can also be provided anonymously via Crime Stoppers on 0800 555 111.