The violence that broke out at Manchester United on Sunday was "completely unacceptable" and should be condemned in "the strongest possible terms", police leaders have said.
Two officers were injured including one who needed emergency treatment for a "significant slash wound to his face" after being attacked with a bottle at Old Trafford, Greater Manchester Police (GMP) said.
The violence happened during protests before United‘s scheduled game against Liverpool, which eventually had to be postponed for safety reasons.
Several hundred fans breached security and stormed the pitch in protest at the club’s American 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 police and horses.
Stu Berry, head of the Greater Manchester Police Federation, said officers were not "punchbags for people protesting".
"At the end of their shifts, hard-working police officers should be able to go home to their families in one piece. Not be rushed to hospital," he added.
Mr Berry said the trouble-makers should be identified and prosecuted but that only a "minority" were to blame.
The chairman of the National Police Federation, John Apter, also condemned the violence.
He said: "Yet again we’ve seen a so-called peaceful protest turn to violence with that aimed at my colleagues. Officers were injured and required hospital treatment. This is completely unacceptable."
GMP assistant chief constable Russ Jackson said it was obvious some people had turned up to cause trouble and condemned their "reckless and dangerous" behaviour.
Sky Sports cameras captured the moment fans made in into the stadium after breaking through barriers and a handful of stewards.
A flare was thrown as supporters roamed the pitch, while a few clambered on the goals and removed corner flags.
There was also a raucous protest by several hundred outside the Lowry hotel, where Manchester United’s stars were waiting to leave for what was meant to be a vital game 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 watching from inside.
The match – which would have seen Manchester City crowned champions had Liverpool won – was eventually called off and is still to be rescheduled.
The Premier League said it understood the "strength of feeling" of fans but said the actions of a "minority" had no justification.
"We sympathise with the police and stewards who had to deal with a dangerous situation that should have no place in football," its statement added.
Speaking to Sky News on Monday, Foreign Office minister James Cleverly also said there was no excuse for violence but that fans’ "frustrations" needed to be recognised.
"I cannot condone the images that we have seen about storming the ground, but we do need to understand the frustrations that fans have, not just with Manchester United but with a number of clubs across the game," he said.
Mr Cleverly said supporters had "to be at the heart of this game" and pointed to the newly announced fan-led review being led by former sports minister Tracey Crouch.
The review is set to look at 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, blamed United owners the Glazer family for the protests and urged them to put the club up for sale.
He said the Super League debacle of a few weeks ago had reignited long-held anger over the Glazers – whom many supports feel are out of touch and only care about profit.
"There’s huge discontent, not just across Manchester United fans, but I think for football fans up and down the country and I think they are just saying enough is enough," said Neville
"The Glazer family have been resilient and stubborn for many, many years. I think they are struggling to meet the financial demands that this club needs and have done for some time."
Latest accounts show gross debt is now £536m. United were debt free before the Glazers took over.
The fallout from the attempted breakaway European Super League has also triggered protests in recent weeks at other clubs who were set to join, including Chelsea and Arsenal.
The project appears dead such was the universal outcry, but the saga has reignited the unease over foreign ownership of clubs and what many fans see as the side-lining of supporters in favour of profit.