Six people were arrested after a banner about Bolton striker Marvin Sordell was shown at Millwall.
It was unveiled in a home section of the New Den stadium during the south London side's 2-1 win over Derby.
Sordell was abused by some Millwall fans when the sides met at the New Den last month, leading to a 13-year-old boy being banned from the ground.
A Metropolitan Police spokesman said the arrests were made over alleged public order offences.
"Six males were arrested during the game at Millwall in relation to a banner being unfurled at the game," he said. "They remain in custody at this time."
The banner was swiftly removed by Millwall staff during the game.
The club declined to comment on the issue, and assistant manager Joe Gallen - standing in for Lions manager Kenny Jackett, who missed the game with flu - also refused to discuss the matter.
Sordell took to Twitter after the Millwall match last month to claim he and team-mates Lee Chung-young, Darren Pratley and Benik Afobe were subjected to racist taunts.
The 21-year-old, who represented Great Britain at the London 2012 Olympics, has also faced subsequent racial abuse on Twitter and Facebook.
The original incident sparked an investigation led by both clubs.
Last week Millwall confirmed the boy in question had admitted abusing Sordell and subsequently issued a written apology, which the player accepted.
Given his age, Millwall decided not to impose a life ban on the youngster and offered him a place on a Millwall for All educational programme, "in the hope that we can change his outlook on equality, racism and life in general".
Sordell also faced racist abuse from Serbia fans while playing for England Under-21s in Krusevac on 16 October.