The forward was abused by a 13-year-old Millwall fan on October 6 which led to the teenager being banned from the club's stadium
The banner was displayed in the home section of the The Den during the club's 2-1 victory over Derby on Saturday but it was quickly removed by staff. The message on the banner was related to an incident on October 6 where Sordell was abused by a 13-year-old Millwall supporter.
The 13-year-old was subsequently banned from the stadium but Sordell has been subjected to racial abuse over Twitter since the incident.
A Metropolitan Police spokesman said: "Six males were arrested during the game at Millwall in relation to a banner being unfurled at the game. They remain in custody at this time."
The 13-year-old Millwall fan, meanwhile, has issued an apology to Sordell and has been offered a place on one of the club's education programmes.
"The individual has been banned from Millwall matches for the foreseeable future, but as a club we also felt, given the boy's age and background, we had a duty to play our part in attempting to educated and rehabilitate him," the club confirmed.