English Championship side Millwall have said they are "dismayed and saddened" after fans booed players that took a knee ahead of their 1-0 defeat to Derby on Saturday.
Professional clubs in England have taken a knee before kickoff in matches to support the Black Lives Matter movement, but the game at The Den was marred by supporters booing players.
The English Football Association condemned the behaviour in a statement on Saturday, and acting Derby County boss Wayne Rooney said it was "disappointing and upsetting" to see.
"Yesterday I witnessed the disgraceful and mindless behaviour by a large section of the Millwall supporters in the stadium," Rooney wrote in a social media post. "Prior to the match, we were aware of the possibility of a planned disruptive response during the taking of the knee in support of the BLM campaign, but nothing prepared us for what we heard.
Further comments from yesterday's game.... pic.twitter.com/7T25MmMDwI— Wayne Rooney (@WayneRooney) December 6, 2020
"I was proud of my players and staff for not letting the fans deter them from continuing to present a positive and important message that taking the knee immediately before a game sends. The professionalism of the players on both sides throughout the game was a credit to our profession after such an incident.
"On behalf of Derby County, I want it to be clear to everyone associated with the club that we represent all sectors of our community, regardless of colour, gender or sexuality."
Millwall released a statement on Sunday that 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."
Millwall's match was among the first in England to welcome fans back to the stadium in the wake of the coronavirus pandemic, which had locked fans out of matches since March.
Derby forward Colin Kazim-Richards, who is Black, described the booing as an "absolute disgrace" in a tweet after the match, where he expressed pride in taking a stand against racism.
"Having to say this is a pain," Kazim-Richards said. "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 [Derby] did too."
Coronavirus regulations in England mean that only 2,000 local fans are allowed to attend games with no away fans permitted to travel.