Five people were taken into custody after an all-out brawl involving up to 100 people erupted at a KFC in Auckland on Monday.

There are reports at least one person was injured in the fight on Massey Rd in Mangere East, in which knives and wood planks were used and people threw chairs at each other.

The fight was between mainly students and at least two of the five people arrested were under 18.

The schools involved in the fight have not been identified, but police say they are investigating. It's not yet known why the brawl broke out.

Police are talking to witnesses and those involved after the incident, which happened shortly after 5pm.

A KFC employee told the New Zealand Herald the students, believed to be from Southern Cross Campus and De La Salle College, rushed into the restaurant and started fighting.

"It was a really big fight between school students. They came down to our store and started punching a guy. They just started smashing him.

"They just came in, they picked up chairs and everything."

The group included both boys and girls, and some appeared to be under 16, the employee said. Some were bleeding and covering their faces with their hands.

Newshub.