The English Football Association is set to launch an investigation after a Leicester supporter punched a Nottingham Forest player in Monday's FA Cup fourth-round tie.
A fan invaded the pitch after Forest's third goal and appeared to land a blow on Keinan Davis, the Aston Villa loanee, before he was taken away by stewards and arrested by Police.
Arrests at football matches are the highest in years and this latest incident, in the first East Midlands derby since 2014, will only increase concerns over rising fan disorder.
Chief Inspector Neil Williams, of Nottinghamshire Police, said: "We have arrested a man in connection with an incident that took place today during the first half of the Nottingham Forest and Leicester City FA cup match.
"Investigations are in the very early stages and our enquiries into the circumstances surrounding the incident are ongoing."
Joe Worrall celebrated in front of the Leicester supporters in the Bridgford Stand, after scoring Forest's third goal with a header, with one fan running onto the field.
As Forest's players celebrated, the fan appeared to land a punch on Davis before he was restrained by Ryan Yates.
The moment Keinan Davis was punched in the head by the violent Leicester City fan. Photo / ESPN
Stewards then pulled the man to the floor before he was led away, but the FA is likely to investigate.
A Leicester City spokesman said: "The Club is appalled by such behaviour and offers its apologies to Nottingham Forest and its players whose safety was compromised. We applaud the swift action taken by City Ground stewards in apprehending the individual involved, who will be subject to a lifetime ban from all Leicester City fixtures at home and away. We will offer every support to the relevant authorities in pursuing further appropriate action."
Before the East Midlands derby, Nottinghamshire Police had urged supporters of both clubs to enjoy the occasion and not "spoil the day for others".
It is the latest incident of crowd trouble in recent weeks, with Bolton's game at Morecambe on Saturday temporarily stopped after an alleged racist remark from a home supporter. Five Bolton fans were later arrested for offences including being drunk and disorderly, pitch encroachment and an assault on a police officer, after their team's equaliser.
A fan also encroached onto the field in Rotherham's game at home to Accrington, kicking the ball away from the spot after a controversial penalty was awarded to the visitors.
Championship side Nottingham Forest went on to win the tie 4-1 in a huge upset that left Leicester's season now in danger of flatlining under Brendan Rodgers.
Having previously eliminated 14-time winner Arsenal, it's an incredible cup run for second-tier Forest, reviving memories of its prowess in the last century before being relegated from the Premier League in 1999.
Forest was champion of England in 1978 and then won back-to-back European Cups. The only titles since then came in the League Cup in 1989 and 1990. Now there will be hope of another deep FA Cup run for the team that reached the 1991 final against Tottenham, with fellow second-tier club Huddersfield next up.