Fighting broke out amongst Millwall fans towards the end of their 2-0 FA Cup semi-final loss to Wigan Athletic at Wembley on Saturday.
Police with batons moved in to quell the trouble in an eerie reminder of the scourge of hooliganism that once plagued the English game.
The fighting, which left many young children in the stands in tears, came on the weekend of the 24th anniversary of the Hillsborough disaster.
One Millwall fan was even highlighted on television, snatching a police officer's helmet and smilingly tucking it inside his jacket.
"I didn't see or wasn't aware of any fighting during the game. I was concentrating on the game and I wasn't aware of any problems," said Millwall manager Kenny Jackett.
"I'd need to examine the facts, see it, and talk to people, before I can give an opinion on it."
Told by reporters that some young Millwall fans had been reduced to tears by the fighting, he said: "I'm very, very sorry if that's the case. But until I do see those images and get the time to sum that up for myself, it's tough for me to comment."
Wigan chairman Dave Whelan said he had been stunned by the violence which left some fans injured.
"I can't understand why the Millwall fans would fight each other," Whelan told BBC Radio.
"I understand if they want to fall out with the visiting team, but why would they fall out amongst themselves? It just gives football a very, very poor reputation.
"We know Millwall are a tough club, their team's tough to play, the supporters are Millwall, Millwall, Millwall... straight through. But don't fight each other. I couldn't understand that."
A Millwall spokesman said the club would release a statement later on Saturday and would investigate the incidents.
Wigan manager Roberto Martinez criticised the feuding fans.
"It is disappointing," said Martinez. "It was an exciting game and there was plenty to concentrate on."
London club Millwall had long been associated with violence and hooliganism in the 1970s and 1980s but have been making strides to combat the problem in recent years.
The incidents sparked an immediate furore on social media.
"As for those morons we've just seen scrapping in the stands, no one likes you, we don't care," tweeted former England striker Gary Lineker, in a reference to a Millwall terrace chant.
Former Wigan striker Jason Roberts also condemned the violence.
"What makes me sad is that it drags the good name of football down," he told the BBC.
"It was a small pocket of people but it looked so bad for our game, which has done well to combat violence in the last 30 years."