Millwall fans who shamed Wembley by fighting a bloody battle among themselves while their team slumped to an FA Cup semi-final defeat will face life bans, the London club warn.
Eleven people were arrested after violent scuffles broke out towards the end of their 2-0 defeat by Wigan Athletic.
Police wielding batons eventually brought the disorder under control, and the Metropolitan Police said four officers had sustained "minor injuries".
"Our position is clear. Anyone associated with our club found guilty of violent behaviour will be banned indefinitely from Millwall matches in addition to any punishment they receive from the authorities," said Millwall chief executive Andy Ambler.
"Having worked so hard to show the positive side of our club both on and off the field, we cannot allow the actions of a mindless minority to undermine that.
"At this stage we are still in the process of establishing the full details of what happened in a section of the ground during the second half. When we have those facts at our disposal and the police have completed their investigations we may be in a position to comment further."
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 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. We have worked very hard, the chief executive and chairman, to do everything we possibly can to be trouble-free. We have had high-profile games that have gone very well. We have done everything we possibly can."
A police spokesman said it was investigating "sporadic disorder amongst fans in the Millwall supporters area of Wembley Stadium". "The policing response was robust and proportionate and 11 people were arrested during the game in connection with the violence that broke out shortly before half time and full time," he said.
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 said.
"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."
Football Association general secretary Alex Horne said that fans involved in the violence could be banned from all grounds in the country.
"The Metropolitan Police and The FA are investigating scenes of sporadic violence and disorder in the Millwall FC end at today's FA Cup semi-final," said Horne.
"The FA and Wembley Stadium will work with police and representatives of Millwall FC to review all events.
"We will look to ensure those involved are identified and we would call for criminal charges and a football banning order to be brought against them.
"The FA deplore the scenes which have taken place, which are unacceptable. Everything will be done to take action against those involved."
Wigan manager Roberto Martinez criticised the feuding fans. "It is disappointing," he said. "It was an exciting game and there was plenty to concentrate on. A small minority give the game a bad name."
British newspapers condemned the violence, with the Sun on Sunday running the simple headline "Scum!" on its website, adding underneath: "Vicious Millwall fans shamed football by turning Wembley into a bloodbath." The Daily Mail website added: "Football shamed as Millwall fans fight each other in the stands at Wembley."
London club Millwall had long been associated with violence and hooliganism in the 1970s and 1980s but have been making strides to tackle the problem in recent years.