Birmingham's Championship match at home to rivals Aston Villa was marred by disgraceful scenes early in the first half as a supporter ran onto the pitch and threw a punch at the back of Jack Grealish's head.
Villa's 1-0 win at St. Andrews was barely 10 minutes old when a fan came onto the field of play and approached Grealish.
The fan then swung a punch from behind and caught the Villa midfielder, who fell to the ground. A steward then managed to restrain the supporter and marched him off the pitch.
West Midlands police confirmed the supporter had been arrested and later released his name, Paul Mitchell, 27, and said he'd been charged with with "invading the pitch and assaulting Aston Villa player Jack Grealish during the Second City derby."
Grealish later scored the winner to make it 1-0 to Villa on 66 minutes.
After the game, Grealish said: "I was unaware [of the fan] at the time. We had a throw in or a corner and I was walking into position and I just felt a whack around the side of the face. Obviously there is rivalry in football, but I don't think there is any place for that.
"I just tried to get on with my job and I think I did -- I scored the winner. It was unbelievable, in front of our fans and after what happened in the first half. It was something I have dreamed of since I was a little kid.
"[It has been the] best day of my life. To come here for first time and captain Aston Villa and score the winner, particularly for me being an Aston Villa fan."
Dean Smith, the Aston Villa manager, said: "It shouldn't happen. Security should be better. Unfortunately some mindless moron has gone on the pitch, attacked Jack and you have 15,000 idiots clapping him."
Birmingham manager Garry Monk labelled it a "disgusting act and an individual whose behaviour in no way represents these fans or the values of this club and the football family as a whole."
An EFL spokesman said: "The EFL condemns the mindless actions of the individual who encroached on to the pitch at St. Andrews on Sunday afternoon. It's a situation no player should ever be faced with.
"In all circumstances the playing surface is for players, not supporters and those playing in the game must be able to do so safe in the knowledge they will not be subjected to this type of behaviour.
"Whilst this incident falls within the remit of the Football Association, we will work with all the relevant parties to address the issue of player and match officials' safety on the pitch and ensure the appropriate action is taken."
Grealish did not require treatment and was able to resume play in the Second City derby, with Aston Villa starting the day in 14th place and Birmingham sitting 10th, five points off the playoffs.
A Football Association spokesperson said: "We strongly condemn the incident in the match between Birmingham City and Aston Villa, which saw a person enter the pitch and assault a player.
"It is a criminal offence to enter the field of play and we will be working with the police, the relevant authorities and the club to ensure the appropriate action is taken."
A Birmingham statement read: "Birmingham City Football Club would like to apologise to Jack Grealish and Aston Villa Football Club for an incident in this afternoon's derby match.
"We deplore the behaviour of the individual who committed this act and rest assured he will be banned from St. Andrew's for life. The Club will also support any further punishment this individual may face in the eyes of the law.
"The Club will be working with the relevant authorities to investigate all the circumstances and we will be reviewing our stadium safety procedures.
"What happened has no place in football or society. Jack is a Birmingham lad and regardless of club allegiance should not have been subjected to this -- there are no excuses.
"Again, we apologise to Jack and all at Aston Villa Football Club."
A statement from Villa read: "Aston Villa Football Club is appalled by the disgraceful attack on Jack Grealish during today's game. A red line has been crossed by this cowardly on-field assault on a player, which is unprecedented in English football."
Sunday's act of violence was not the first time the Midlands derby had witnessed a flash point. In 2002, a fan was jailed for four months after entering the field of play and taunting Peter Enckelman, the Aston Villa goalkeeper who had conceded a goal in Birmingham's 3-0 victory at St. Andrews.
Michael Harper, 26, from Solihull, was sentenced at Birmingham Magistrates Court after admitting to running onto the pitch before the clash with Enckelman.