A sign directed at Bolton striker Marvin Sordell, who was verbally abused by one of the club's fans last month, was unveiled at The Den during the Lions' 2-1 Championship win against Derby on Saturday.
The banner, which read: "Sordell is a c*nt," attacked Sordell for reporting that he had been racially abused by Millwall fans while warming up for a match on October 6. Sordell's allegation led to a 13-year-old boy being banned from Millwall's ground.
A Metropolitan Police spokesman said six men were arrested on suspicion of public order offences and have been bailed until early December.
The banner was swiftly removed by Millwall staff. The club have refused to comment on the issue, and assistant manager Joe Gallen - standing in for manager Kenny Jackett, who was ill - opted not to discuss it.
After last month's match Sordell said on Twitter that he and team-mates Lee Chung Yong, Darren Pratley and Benik Afobe had been subjected to racist taunts.
Both clubs led an investigation and Millwall confirmed last week that the boy had admitted abusing Sordell and issued a written apology which the player accepted.
Given his age, Millwall have not imposed 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."