Millwall have described themselves as "dismayed and saddened" after supporters at The Den jeered a demonstration in support of Black Lives Matter prior to Sunday's (AEDT) game against Derby County.
Fans are coming back to stadiums in some parts of England - including London - this week amid the coronavirus pandemic.
But their return for the Championship clash between Millwall and Wayne Rooney's Derby was marred by the loud boos that met the players' now customary anti-discrimination display as they took a knee.
The act, popularised in the United States by NFL quarterback Colin Kaepernick, has remained part of elite competitions across the world since the death of George Floyd in May.
Players are demonstrating in support of BLM and against racial injustice and police brutality, but these protests have taken place behind closed doors in English football until now.
A statement from Millwall, who lost 1-0, 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 Football Association spokesperson said on Sunday: "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."