De Klassieker was suspended for the second time after Davy Klaassen was struck by an object from the crowd, Voetbal International reports.
Pictures of the match, shown live on ESPN in the Netherlands, show the Ajax midfielder bleeding after the object struck him halfway through the second-half.
It is the second time the KNVB semi-final was delayed after the match was briefly suspended just seconds after kick-off due to fireworks.
Former Danish international and Ajax winger Kenneth Perez watched the events between Feyenoord and Ajax during De Klassieker with disbelief.
Perez, appearing as an analyst on ESPN could not believe their eyes when the game had to be stopped for the second time. Klaassen, who had just given the visitors the lead, had to leave the field with a bleeding head.
“Act normally. You can’t just throw everything, can you? Now someone has hit Klaassen with a lighter. This is not normal,” Perez said.
“Those players too, act normal. It just got bloody dangerous. That’s bad right?! But there are several things, including that fireworks in the first minute.”
A gigantic fireworks storm halted the heated duel in De Kuip in the first minute. Perez was surprised the fans were allowed to produce such a display considering the laws against such displays.
He added:“I don’t think you’re allowed to bring fireworks into the stadium. These weren’t small rocks either. We have a strict football law, but it is not enforced at all. Then you can write down anything you want, but what’s the point? Those fireworks are unbelievable. It’s just getting ridiculous to have to stand there.
“I can imagine that the police are done with it.
“There comes a time when people say, sort it out. The police could be better deployed. We’re done. Stop with that sixth right back, let the clubs put the money into security, because this is getting way too far out of hand.”