Greater Manchester Police have launched an investigation after an officer was left needing emergency hospital treatment following disturbances which led to the postponement of Manchester United's Premier League clash with Liverpool at Old Trafford.
Fans broke into the stadium and invaded the pitch in protest at club owners the Glazer family, while outside the ground bottles and barriers were thrown at police officers and horses.
Two officers were injured, with one "attacked with a bottle and sustaining a significant slash wound to his face, requiring emergency hospital treatment", GMP said.
Assistant chief constable Russ Jackson said it was clear many demonstrators had no intention of protesting peacefully as he condemned their "reckless and dangerous" behaviour.
He added: "The actions of those today required us to take officers from front-line policing and call in support from neighbouring forces to prevent the disorder getting worse. At different points, bottles and barriers were thrown, officers assaulted and people scaled the stadium structure creating risk for themselves and officers."
The Red Devils were among 12 clubs that last month signed up for the breakaway European Super League, which collapsed within 48 hours due to huge, unrelenting pressure.
Those plans brought anger against the already despised Glazer family to a new level, with fans congregating at both Old Trafford and the Lowry, the team hotel in the city centre, to demand change ahead of Sunday's clash with Liverpool.
Police said that by late afternoon around 200 protesters had gathered outside the Lowry and over 1,000 at the stadium.
There was an initial unspecified delay to the scheduled 1630 kick-off before confirmation came through from United at 1735 that the match had been postponed "due to safety and security considerations around the protest".
A club statement read: "Our fans are passionate about Manchester United, and we completely acknowledge the right to free expression and peaceful protest.
"However, we regret the disruption to the team and actions which put other fans, staff, and the police in danger.
"We thank the police for their support and will assist them in any subsequent investigations."
The Premier League said it understood the fans' "strength of feeling", but condemned "all acts of violence, criminal damage and trespass, especially given the associated Covid-19 breaches".
It added: "Fans have many channels by which to make their views known, but the actions of a minority seen today have no justification.
"We sympathise with the police and stewards who had to deal with a dangerous situation that should have no place in football. The rearrangement of the fixture will be communicated in due course."
Liverpool, another founding member of the Super League, said they were in "full agreement" with the postponement.
Andy Burnham, mayor of Greater Manchester, said on Twitter: "It is important to make clear that the majority of supporters made their protest peacefully today. However, there is no excuse for the actions of a minority who injured police officers and endangered the safety of others.
"This could be an important moment to change football for the better. We should all condemn violence of any kind and keep the focus on the behaviour of those at the top of the game."
Manchester United Supporters Trust wants the Government to act to prevent single private shareholders holding majority ownerships in football clubs.
It said in a statement: "On the back of the indefensible ESL proposals, and an 'apology' from the Glazers which we do not accept, we need to give fans a meaningful share in the ownership of United and a meaningful voice in how it is run.
"The Government now needs to act. That has to mean a process which results in fans having the opportunity to buy shares in their club and more to the point no single private shareholder holding a majority ownership of our football clubs which allows them to abuse that ownership."