A man has been arrested following an incident in which a fan ran onto the pitch in front of Nottingham Forest players as they celebrated their third goal in a 4-1 FA Cup win over local rivals Leicester City.
The supporter was restrained by stewards and then escorted away from the City Ground Stadium by police.
It was the first meeting between the East Midlands' clubs since both were in the Championship in 2014.
Leicester City say they are "appalled".
Nottinghamshire Police chief inspector Neil Williams told BBC Sport: "We have arrested a man in connection with an incident that took place during the first half of the Nottingham Forest and Leicester City FA Cup match.