Police are seeking witnesses to a serious assault that has left one person in critical condition.

An 18-year-old man has been arrested after assaulting another 18-year-old man outside a bar in Hamilton on Friday night.

The victim is in critical condition in Waikato hospital following the assault that occurred at approximately 2am near Biddy Mulligans Bar on Hood Street.

"It’s important to realise just how dangerous any blow to the head can be,' says CIB Waikato Detective Senior Sergeant Scott Neilson.

"It appears the incident has occurred as a result of an argument between a group of people earlier in the night.

"The victim has been approached by the offender and the assault has occurred after the pair have exchanged words."

Police would like to hear from anyone who witnessed the incident or has any information about it.

Contact Police via 105, either by calling or going online to  https://www.police.govt.nz/use-105 using ‘Update Report’.

The Waikato man who was arrested will appear on assault charges in the Hamilton District Court.