Videos circulating on social media showed a significant number of supporters booing, with Kick It Out condemning the behaviour of those fans booing the gesture, and admitting they are “saddened”.
A statement from Kick It Out read: "We are saddened by the behaviour of fans booing the players taking the knee today at Millwall. What this demonstrates is that players are right to continue standing up to discrimination, whether that is through taking the knee or speaking out.
“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.”
Only 2,000 spectators were allowed inside the ground due to London being placed in tier two of the Government's tier system following the end of the national lockdown on Wednesday.
Ahead of the Sky Bet Championship match, the topic of players taking a knee was discussed on Millwall's fans forum and a statement from the first-team squad was published on the official club website on Friday.
It read: "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.
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"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.
A section of Millwall fans booed as their players and Championship opponents Derby took a knee in support of the Black Lives Matter movement at the start of their match pic.twitter.com/o1wcLOoGar
— Football Daily (@footballdaily) December 5, 2020
"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.
"We will continue to do this until the start of the New Year when a new and comprehensive anti-discrimination strategy will be announced by the club."
Millwall have been approached for comment.
Additional reporting by PA