Millwall say they are "dismayed and saddened by events which marred Saturday's game against Derby County" after a section of home fans booed players taking a knee ahead of kick-off.
The club released an official statement on Sunday condemning the incident and say club representatives will meet with Kick It Out and representatives from other appropriate bodies "in an attempt to use Saturday's events as a catalyst for more rapid solutions".
"The club has worked tirelessly in recent months to prepare for the return of supporters and what should have been a positive and exciting occasion was completely overshadowed, much to the immense disappointment and upset of those who have contributed to those efforts," the statement said.
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"The impact of such incidents is felt not just by the players and management, but by those who work throughout the club and in its Academy and Community Trust, where so many staff and volunteers continue passionate endeavours to enhance Millwall's reputation day after day, year after year.
"The club will not allow their fine work to be in vain.
"The players are continuing to use the biggest platform they have to support the drive for change, not just in football but in society generally.
"There is much work to be done and at Millwall everyone is committed to doing all that is possible, both individually and collectively, to be a force for good and to ensure that the club remains at the forefront of football's anti-discrimination efforts."
"Over the coming days, club, Academy and Community Trust staff will meet with Kick It Out and representatives from other appropriate bodies in an attempt to use Saturday's events as a catalyst for more rapid solutions which have an impact both in the short and long-term.
"Further comment will be made once those meetings and discussions are concluded."
Players in England's top divisions have been taking a knee at the start of matches since football resumed in June as part of global sporting protests against racial injustice.
Saturday's match, which Derby won 1-0, represented the first time Millwall supporters had been able to attend a home game since the coronavirus pandemic saw the Championship halted in March.
Millwall defender Mahlon Romeo has accused his own supporters of "spreading hatred" and said he felt personally disrespected.
"Today's game, to me now, has become irrelevant," he told the South London Press. "The fans have been let back in - which the whole team was looking forward to. But in society there is a problem - and that problem is racism.
"The fans who have been let in today have personally disrespected not just me but the football club. And what the football club and the community stand for. What they've done is booed and condemned a peaceful gesture which was put in place to highlight, combat and stop any discriminatory behaviour and racism. That's it - that's all that gesture is.
"And the fans have chosen to boo that, which for the life of me I can't understand. It has offended me and everyone who works for this club - the players and the staff.
"I'm speaking on behalf of myself here - not any of the other players - I want to make that very clear. This is the first time I feel disrespected. Because you have booed and condemned a peaceful gesture which - and it needs repeating - was put in place to highlight, combat and tackle any discriminatory behaviour and racism in general."
Derby forward Colin Kazim-Richards, who has opted to stand while raising a fist as opposed to kneeling at the start of games, described the incident as an "absolute disgrace".
But having to say this is a pain but I’ll say it every single damn time this is why I STAND and STAND PROUD and I have to say every single person involved with @dcfcofficial did too made me proud to wear this shirt with the boys today!!! Absolute disgrace.. pic.twitter.com/lVsdb1KUpa— Colin Kazim-Richards (@ColinKazim) December 5, 2020
He wrote on Twitter: "First of all so proud of my team @dcfcofficial the guys all of us stuck together and grinded out a massive result!! Everyone at the club has bought in and I'm sure we going to see the results!!
"But having to say this is a pain but I'll say it every single damn time this is why I STAND and STAND PROUD and I have to say every single person involved with @dcfcofficial did too made me proud to wear this shirt with the boys today!!! Absolute disgrace.."
Millwall boss Gary Rowett was left frustrated that the return of fans had been overshadowed.
He told Sky Sports: "I'm disappointed that we are talking about that when we should be talking about the fact we are all back and we want to enjoy the football match again.
"The club do an enormous amount of work on anti-racism and the club do a lot of work in the community and there is some really positive stuff, so of course I am disappointed."
On players taking a knee, Rowett added: "Is it a political message, is it an anti-discrimination message? The players have come out and said they don't support the political aspect, but they do support the anti-discrimination aspect of it and of course we all do."
Derby interim boss Wayne Rooney, who secured his first win in charge of the Rams, said: "With everything that has been going on in recent months, it was very surprising.
"I don't want to say much about it, but all I can say is everyone at Derby County Football Club, we obviously took the knee, and no one condones that behaviour."
Millwall Supporters' Club: Motives behind the booing weren't racist
Millwall Supporters' Club released the following statement on Sunday, which claimed that the supporters who booed while players knelt prior to kick-off were not being racist, defended the club's past efforts to tackle discrimination, and stated that the incident highlighted the need for greater "clarity and understanding on both sides of the debate".
It read: "Yesterday's game against Derby County was a testing day for Millwall FC and those associated with it.
"We fervently believe that the motives of those behind the booing were not racist. However, at a time of heightened awareness and with the country watching, the choice of those individuals was always going to damage their club and be perceived by the media as racist.
"The greatest thing it highlighted is the need for clarity and understanding on both sides of this divide.
"Anyone who believes it was a racist act, should read the views of those who booed and see they were doing it in reaction to the war memorials and statues of Churchill defaced by the BLM organisation and the extreme political views they hold, and for which 'taking the knee' is associated with.
"These same fans have never booed the Kick it Out campaigns on our pitch or the huge work of the Millwall Community Trust and its many anti-racism campaigns.
"Equally, anyone who booed in the ground yesterday should read the views of Mahlon Romeo and those of the Millwall players released on Friday.
"They chose to 'take the knee' to highlight the need for more anti-discrimination work and action, something Millwall has always been at the forefront of through our community work.
"They explicitly did not use it to support any political viewpoint or organisation and therefore the booing shows disagreement with anti-discrimination.
"It is within our Millwall DNA to make The Den a hostile environment for the opposition players, not our own. We never want that to be sanitised or taken away as it is what makes us unique.
"But…if you are unable to create a hostile atmosphere at The Den without resorting to racist, homophobic or other discriminatory language or actions then you should make the decision not to attend."
Kick It Out: The fight continues
The English Football League (EFL), Football Association (FA) and anti-discrimination charity Kick It Out all released statements in the aftermath of the match condemning the incident.
"We are disappointed that a small group of supporters have today chosen to voice their opposition to such activities directly aimed at raising awareness of the fight against racism," the EFL statement said.
"Discrimination in any form is not welcome and we remain committed to working with our clubs, including Millwall who undertake a significant amount of work on equality and inclusion initiatives, as we continue with our collective objective to eradicate all types of prejudiced behaviour, ensuring the EFL is an inclusive and diverse environment for all."
A statement from the FA said: "The FA supports all players and staff that wish to take a stand against discrimination in a respectful manner, which includes taking of the knee, and strongly condemns the behaviours of any spectators that actively voice their opposition to such activities."
Kick It Out said it was "saddened" by the scenes at The Den.
"What this demonstrates is that players are right to continue standing up to discrimination, whether that is through taking the knee or speaking out," a Kick It Out statement said.
"The fight for racial equality continues and we will continue to work closely with clubs across the country to tackle discrimination in all its forms.
"We applaud the players for taking a stand and defying the hate shown today."
Millwall's players had released a statement on Friday signalling their intent to continue taking the knee before matches until the New Year, at which point "a new and comprehensive anti-discrimination strategy" is to be announced by the club.
"As a squad we are fully supportive of the entire football family's efforts in ridding the sport, and society generally, of all forms of discrimination," the statement from the club's first-team squad said.
"It is our duty as players to reinforce the positive messaging and action of clubs, Community Trusts, charities and governing bodies, and we do so with great pride and knowledge that so much good work is being done up and down the country.
"The gesture of 'taking the knee' before matches provides an opportunity for us to do exactly that and continues to allow all those playing to publicly showcase their support - on behalf of the whole squad - for the fight against discrimination.
"We wish to make clear that taking the knee, for us, is in no way representative of any agreement with political messaging or ideology. It is purely about tackling discrimination, as has been the case throughout."
Footballers all across the world began taking a knee last season to show their support for the ongoing anti-racism movement, which surged following the death of George Floyd in May who died after police officer Derek Chauvin knelt on his neck for eight minutes and 46 seconds.
Players decided to continue the gesture as the 2020-21 season began in September, with the Premier League and English Football League (EFL) reiterating their support for players who chose to do so.
Following the restart of the 2019/20 campaign, Premier League players decided to wear 'Black Lives Matter' on the back of their shirts instead of their names for the first round of fixtures, and then had a Black Lives Matter logo on their shirts for the remainder of the season.
This has been replaced by a 'No Room For Racism' sleeve badge for the new season, but Premier League managers such as Crystal Palace boss Roy Hodgson have stressed that both messages remain vitally important.
Meanwhile, players in the Championship, League One and League Two have been wearing a 'Not Today or Any Day' logo on their shirts for 2020/21 in recognition of their own fight against racism.
Government minister: Black Lives Matter a political movement, different to racial equality
The UK Government's Environment Secretary George Eustice has told Sky News that Black Lives Matter is a "political movement" as he faced questions about the fans who booed players at The Den.
"The issue of race and racial discrimination is something that we all take very, very seriously," Mr Eustice said.
"My personal view is that Black Lives Matter, capital B, L and M, is actually a political movement that is different to what most of us believe in, which is standing up for racial equality.
"Each individual can take their own choices about how they reflect this. I know a number of people feel quite strongly and have taken that approach.
"I'm afraid I'm listening on a sort of a walkie-talkie in the middle of a street with a bus going behind me so… there have been problems with racism in football in the past," he replied.
Mr Eustice was played footage of the booing and asked for his reaction to what he had seen and heard.
"It's right that it's called out and challenged when we see it.
"It doesn't have any place in society today and if people choose to express their view in a particular way that should always be respected."
A spokesperson for Mr Eustice later told Sky News that when talking about respecting people's right to express themselves, he was speaking about players taking a knee.