West Bromwich Albion’s FA Cup defeat against local rivals Wolves was suspended for 38 minutes because of fan disorder at The Hawthorns.
Trouble flared after the Premier League side went 2-0 up in the 78th minute.
Fights broke out in the corner of the West Stand and Birmingham Road end while missiles were thrown elsewhere as fans spilled on to the pitch, so the players were led to the dressing rooms.
Police said two arrests were made and a man is in hospital with head injuries.
The Football Association said it has launched an investigation into the incidents, describing the disorder as “completely unacceptable”.
“Safety and security are of the utmost importance, and the behaviour of those involved is dangerous and inexcusable,” said the FA in a statement.
“We will be investigating these serious incidents and the appropriate action will be taken.”
Police worked to restore order so the players could return to the field shortly after 13:50 GMT to begin a warm-up, with the game resuming 10 minutes later.
“We’ve had extra officers at this local derby and they responded immediately as disorder in the stands caused fans to spill onto the pitch,” said West Midlands Police.
“Two people were arrested for public order offences. Another man was arrested before the game for possession of an offensive weapon.”
Matheus Cunha doubled Wolves’ lead, given to them by Pedro Neto in the first half, prior to the start of the disorder.
There were no further goals as Wolves clinched a place in the fifth round, the draw for which was delayed until half-time in Liverpool’s match at home to Norwich after the game at the Hawthorns overran.
Tension was growing before the game was stopped, with a beer bottle thrown towards Wolves’ Tommy Doyle as he took a corner.
Further missiles were thrown in the Smethwick Road end following Cunha’s goal as trouble also erupted at the opposite end of the ground.
Players remained on the pitch for a number of minutes before referee Thomas Bramall eventually led them back to their dressing rooms.
Some members of the West Brom squad, who were concerned for family members seated close to the trouble, went in to the stand to remove their children.
More to follow.
