Two officers were injured including one who needed emergency treatment for a “major facial injury” after being attacked with a bottle at Old Trafford, Greater Manchester Police (GMP) said.
The violence occurred during protests before UniLiverpool’s scheduled game against Liverpool, which eventually had to be postponed for security reasons.
The FA told Sky Sports News they are reviewing the events leading up to the report and are liaising with the club, the Premier League and the security authorities.
Images have been posted on social media showing the injured officer with a cut to his cheek.
Writing on Twitter, GMP Superintendent JP Ruffle said the footage showed an officer from the force’s specialized operations team.
“Absolute shame, disgusting scenes,” he wrote.
“No officer should under any circumstances be subjected to such vile and unnecessary violence. “
Several hundred fans breached security and stormed the pitch to protest the club’s US owners.
Outside the stadium, police said there were more than 1,000 supporters. Most were protesting peacefully, but a group started throwing bottles and barriers at the police and horses.
Stu Berry, chief of the Greater Manchester Police Federation, said the police were not “punch bags for protesters”.
“At the end of their shift, hard-working police officers should be able to get home in one piece. Not to be taken to hospital, ”he added.
Mr Berry said the troublemakers should be identified and prosecuted, but only a “minority” was to blame.
The president of the National Police Federation, John Apter, also condemned the violence.
He said: “Once again, we have seen a so-called peaceful protest turn into violence with the one targeting my colleagues. Officers were injured and had to be hospitalized. It is totally unacceptable. “
GMP Deputy Chief Constable Russ Jackson said it was evident some people had come forward to cause trouble and condemned their “reckless and dangerous” behavior.
Sky Sports cameras captured the moment fans entered the stadium after passing through barriers and a handful of stewards.
A flare was fired as supporters scoured the pitch, while a few climbed on goals and removed corner flags.
There was also a noisy protest of several hundred people outside the Lowry Hotel, where the Manchester United stars were waiting to leave for what was supposed to be a vital match against their fierce rivals.
Manchester Evening News said fans were chanting ‘we decide, we decide, we decide when you can play’ – as some players were pictured looking inside.
The game – which would have seen Manchester City crowned champions if Liverpool had won – was ultimately called off and still has to be rescheduled.
The Premier League said it understood the “strength of sentiment” of supporters but said the actions of a “minority” had no justification.
“We sympathize with the police and the stewards who have had to face a dangerous situation which should not have its place in football,” added his statement.
Speaking to Sky News on Monday, Foreign Secretary James Cleverly also said there was no excuse for the violence, but fans’ “frustrations” had to be acknowledged.
“I cannot tolerate the images we saw of storming the ground, but we have to understand the frustrations of the fans, not just with Manchester United but with a number of clubs throughout the game,” did he declare.
Mr Cleverly said fans need to “be at the heart of this game” and stressed that the recently announced fan-run review was led by former Sports Minister Tracey Crouch.
The review will focus on how fans are treated, as well as issues of governance, financial sustainability and ownership.
Manchester United legend Gary Neville, who was inside the stadium for Sky Sports, United owners blamed the Glazer family for the protests and urged them to put the club up for sale.
He said the Super League debacle a few weeks ago rekindled long-standing anger against the Glazers – who many supporters deem out of touch and only care about profit.
“There is huge dissatisfaction, not just among Manchester United fans, but I think for football fans all over the country and I think they are saying just enough is enough,” Neville said.
“The Glazer family has been resilient and stubborn for many, many years. I think she is struggling to meet the financial demands that this club needs and has been for some time. “
The latest accounts show gross debt is now £ 536million. United were debt free before the Glazers arrived.
Fallout from the European Super League breakaway attempt has also sparked protests in recent weeks at other clubs due to join, including Chelsea and Arsenal.
The project appears to be dead, such was the universal outcry, but the saga has rekindled unease over foreign club ownership and what many fans see as supporters being pushed aside in favor of profit.