Greater Manchester Police have confirmed that six officers were injured as they attempted to control the fan protests throughout Manchester.
Hundreds of supporters staged a demonstration outside Old Trafford before they broke into the stadium.
Incidents also occurred outside the Lowry Hotel in the city centre where the United players were staying before their clash with Liverpool, which eventually had to be postponed.
Protests were generally peaceful but a minority of fans engaged in violent scenes that the GMP have labelled "absolutely atrocious".
A full statement read: "An investigation is on-going into the violent disorder surrounding protests at Old Trafford yesterday which saw a number of officers injured.
"Police were aware of a planned protest at Old Trafford yesterday (Sunday 2 May) and were working closely alongside partners to ensure those that attended were kept safe whilst enabling their right to peaceful protest.
"By late afternoon 1,000 protesters had gathered at Old Trafford and 200 separately at the Lowry Hotel in Salford where the United players were staying.
"Officers continued to monitor the situation and engage with those present, however they were met with hostility as flares were let off and bottles and barriers were thrown at police officers and horses.
"Six police officer injuries have been reported so far with one officer receiving a fractured eye socket, requiring medical treatment, and another sustained a wound to his face, following bottles and cans being thrown from the crowd.
"A third officer was dragged and kicked. The officer who attended hospital has since been discharged."
GMP Assistant Chief Constable Nick Bailey, said: "The behaviour shown at this protest was absolutely atrocious.
"Officers were just trying to do their job and facilitate a peaceful protest, however a number of those present became hostile and aggressive towards officers and forced entry to the football grounds, making it very clear that this protest was not peaceful and ruining it for the majority of protesters who had not intended for the protest to become violent.
"Our officers tried to engage with protestors, but were met with violence and aggression which resulted in enforcement action being taken. Enforcement will always be a last resort, but in these circumstances it was deemed necessary in order to maintain safety during a situation that was rising in hostility.
"No officer should have to come to work and face these conditions. This violence was completely unnecessary and resulted in officers being taken from front line policing and neighbouring forces to prevent the disorder getting worse, putting increased pressure and strain on our service and ultimately taking away resources from those who may be in desperate need of our help."
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