The incident occurred in the ninth minute of the match when a man wearing a flat cap ran onto the field from the Tilton End and punched Grealish in the head from behind.
Grealish fell to the ground but was not badly hurt in the incident and a steward, helped by both sets of players, helped to restrain the man.
Aston Villa captain Jack Grealish has been assaulted by a man during the Birmingham derby
A man wearing a flat cap ran onto the field in ninth minute from the Tilton End at St Andrew’s
The man sprinted onto the field in the ninth minute and struck Grealish with some force – to cheers from some in the home support.
The midfielder fell to the floor but did not appear to be hurt. The intruder was quickly restrained by stewards as several other Villa players rushed in to confront him.
The Aston Villa bench were all on their feet to check on Grealish while Birmingham City players came over to apologise to the Villa captain.
At half-time, Aston Villa coach John Terry was first over to check on Grealish at along with manager Dean Smith speaking to him too.
On the way down the tunnel, Grealish was subject to further goading home supporters nearby, chanting ‘one punch and he knocked him out.’
He was led away by police, blowing kisses to the crowd as he left.
West Midlands Police announced an arrest had been made following the incident.
A post on the force’s official Twitter feed read: ‘ARREST: Following the incident on the pitch involving an attack on an Aston Villa player; a man has been arrested and en route to a custody block. Further details to follow.’
The incident followed a feisty opening to the game, in which Birmingham’s Maikel Kieftenbeld was booked for scything down Grealish, while there were also clashes between fans outside the stadium before kick-off.
Police had to hold back fans while at least one man was seen being treated by paramedics after suffering a head wound.
Fans and players have been reacting with shock at the incident on social media.
Former Manchester United defender Gary Neville called for a heavy punishment on Twitter. He wrote: ‘The club are going have to take a huge punishment for this to act as a deterrent in the future . A Points deduction or Empty stadium for 10 games !’
Gary Lineker branded the attack ‘abhorrent’, adding on Twitter: ‘Well done @JackGrealish1 for keeping his cool.’
West Ham midfielder Declan Rice was quick to react to the incident, tweeting: ‘Absolutely disgraceful what just happened to Jack Grealish’.
Brighton defender Shane Duffy added: ‘That is a disgrace.’
Raheem Sterling said: ‘Idiot of the year… congrats on your lifetime ban.’
More to follow.