A section of Millwall fans booed as their players and Championship opponents Derby took a knee in support of the fight against racial injustice at the start of their match on Saturday.
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.
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."
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.
Millwall defender Mahlon Romeo has accused the club's supporters of "spreading hatred" after a section of fans booed when the players took a knee.
In an impassioned interview after the game, Romeo, a London-born black player who has represented Antigua and Barbuda at international level, explained how the reaction of the club's supporters had affected him.
Romeo told the South London Press: "Today's game, to me now, has become irrelevant. 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."
Harriott 'disappointed' at 'booing' before Colchester match
Callum Harriott expressed his disappointment in a tweet following Colchester's match against Grimsby on Saturday after claiming he heard booing while players took a knee.