The referee took the players down the tunnel, before Ajax went on to win 2-1.

Police later said a 32-year-man had been arrested.

Feyenoord assistant John de Wolf had come out during the 28-minute suspension in play to urge home fans to "use your common sense" before the game got back under way.

Klaassen, 30, was substituted shortly after the resumption, gesturing that he was suffering from dizziness.

"There was some sort of altercation and I went to try to calm things down. Then something hit my head. In the beginning it just hurt. That quickly made place for anger. I thought to myself, 'how is this possible?'," Klaassen told the Ajax website.

"I had a throbbing feeling around the wound and I also had trouble focusing, so I could have stayed on the pitch but that was not going to help me nor the team.

"You don't want to let it get to you but sometimes you have to put your pride aside and do what's best for the team."

The game had also started five minutes late due to heavy black smoke from fireworks, before it was stopped for 21 seconds after kick-off to allow the air to clear.

Dusan Tadic put Ajax ahead, before Santiago Gimenez levelled for the hosts.

Despite the incident and an injury-time red card for Ajax team-mate Kenneth Taylor, Klaassen's goal set up a final against PSV Eindhoven on 30 April.