A 29-year-old man has been arrested in connection with a tweet sent to Derby County player Duane Holmes.
He is being questioned on suspicion of racially aggravated harassment in relation to the comment.
Nottingham Forest have banned a supporter who aimed "horrific abuse" at Derby midfielder Duane Holmes on social media.
Holmes highlighted a racist message he received on Twitter shortly after his side's 2-1 victory at Huddersfield in their opening Sky Bet Championship game on Monday night.
The 24-year-old posted a Tweet celebrating Derby's win, which attracted an offensive reply. The reply was then quoted by Holmes along with a message which said: "I'll just leave this one here."
Derby's East Midlands rivals Forest condemned the Tweet, which it appears was sent by one of their fans, and has banned the individual from attending matches while insisting the club will take "further action" following police inquiries.
The club are aware of some racist social media messages which have been posted by an individual who claims to be a supporter and denounces the conduct of the individual responsible in the strongest possible terms.
The club say they will continue to co-operate with the police who are pursuing their enquiries: "Whilst these enquiries are ongoing the club has suspended the ability of the individual concerned from attending matches.
"In addition to this the club would like it to be known that it will take robust steps and sanction any individual who seeks to assist the individual concerned from attending matches in breach of the club's suspension.
"The club will take further action against the individual when the outcome of the police enquiries and the criminal process is known."