Millwall have insisted they are "dismayed and saddened" by events at The Den on Saturday where players from their own team were booed for taking a knee.
The long-awaited return of supporters was overshadowed by an incident ahead of the Lions clash with Derby in the Sky Bet Championship, which the visitors won 1-0.
Fans were back at Millwall for the first time since February 29 after London was placed in tier two of the current system of coronavirus restrictions, but some of the 2,000 home crowd booed when players from both sides took a knee.
Ahead of the match, the whole squad released a statement on Friday which explained how they would continue to do the gesture in support of the fight against discrimination and not as a political gesture.
Despite this, when both players decided to take a knee in support of the Black Lives Matter movement, it was roundly booed by a large section of those in attendance.
On Sunday morning, a statement from the Championship club read: "Millwall Football Club was dismayed and saddened by events which marred Saturday's game against Derby County at The Den.
"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 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."
A similar incident occurred at the JobServe Community Stadium when Colchester hosted Grimsby in League Two and claimed a 2-1 win.
Before the 5.30pm kick-off got under way on Saturday in front of a crowd of close to 1,000 fans, both sets of players took a knee and a small section of those in attendance booed.
A statement from the League Two club read: "Colchester United are fully behind any and all of our players and staff who take a stand against any form of discrimination in football, sport and life.
"We also condemn the behaviours of any supporters that actively voice opposition to those activities."
