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 2-1 win over Derby.
The banner related to complaints made by Sordell after he was abused by fans while warming up for a match on 6 October, leading 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 on display was swiftly removed by Millwall staff.
The club has so far refused to comment on the incident, and assistant manager Joe Gallen - standing in for manager Kenny Jackett, who missed the game with flu - also opted not to discuss the matter.