West Bromwich Albion's FA Cup defeat by local rivals Wolverhampton Wanderers was suspended for 38 minutes because of crowd trouble at The Hawthorns.
Trouble flared after the Premier League side went 2-0 up in the 78th minute.
Fights began in the corner of the West Stand and Birmingham Road end while missiles were thrown elsewhere and fans spilled on to the pitch, leading to the players going 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 appropriate action will be taken."
West Brom said they "condemn in the strongest terms the unsavoury scenes" and that they would work with West Midlands Police and the FA to help their investigations.
The Championship club added that "any individual involved in the disorder will be subject to a club ban".
Once police had restored order the players returned 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."
How trouble unfolded at The Hawthorns
Tension had been growing before the game was suspended, with Wolves supporters lighting flares after Pedro Neto's first-half opener and then a beer bottle being thrown towards Wolves' Tommy Doyle as he took a corner after the break.
After Matheus Cunha doubled Wolves' lead, further missiles were thrown in the Smethwick Road end as trouble then also began at the opposite end of the ground.
Players remained on the pitch for a number of minutes, with some from West Brom going into the stand to remove their children, before referee Thomas Bramall eventually led them back to their dressing rooms.
"A lot of the players were distressed because that's where their families sit," said Baggies captain Jed Wallace. "They were worried about their children, that's why you could see some of them running over. No-one wants to see that in football."
West Brom manager Carlos Corberan later confirmed none of his players' family members had been harmed.
A supporter was escorted out of a home section with blood streaming down his face as medical staff tried to stem a wound to the top of his head, while another fan was taken out of the stadium on a stretcher, although he appeared to be in good spirits.
Fans were told to return to their seats on numerous occasions and Corberan, together with Wolves manager Gary O'Neil, discussed the situation with stadium officials and police representatives before the teams returned.
There were no further goals as Wolves clinched a place in the fifth round, where they will host fellow Premier League side Brighton. The draw was delayed until half-time in Liverpool's match at home to Norwich as a result of the game at The Hawthorns overrunning.
Reaction to West Brom-Wolves crowd trouble
West Brom manager Corberan told ITV he was "totally disappointed" by the disorder.
"Unfortunately the game was interrupted by this incident and now we are unable to talk about the atmosphere our fans created, we can't talk about this aspect," he said.
"Everything before the incident was unbelievable, between both clubs it was very special. Incidents like that we need to avoid."
Wolves boss O'Neil agreed it had been "a good tie with a good atmosphere" before his side doubled their lead.
"How people behave at football is really important and we need to look at that, make sure everyone is safe," O'Neil told ITV.
"When we came back out the atmosphere had gone, it was really sad to see."
He later added: "Obviously it's really disappointing. Anybody that wants to come to watch a football match should feel safe.
"Young children or elderly people could be in that area and we shouldn't have to discuss incidents like this.
"They shouldn't happen and I hope everyone's safe and there are no serious injuries and there are no lasting effects."
O'Neil added his players were happy to come back out to finish the match after being assured by police and safety officers that it was safe to do so.
"I don't think I've played in a game that's been stopped for crowd trouble before so it was a new one," he said.
"Hopefully I don't have to be involved in one ever again."
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