Greater Manchester Police have condemned the protests which led to the postponement of yesterday's Premier League clash between Manchester United and Liverpool.
The game was eventually postponed after supporters gained entry to Old Trafford and invaded the playing pitch in the hours before kick-off.
Demonstrations also took place outside the Manchester United base at The Lowry Hotel in Salford.
It had been anticipated that the game would go ahead with a delayed kick-off time once police managed to curtail the protests.
However, in a statement, Manchester United said the club had consulted with the authorities, including police, and the decision to postpone was taken in light of 'safety and security considerations.'
In a statement, Greater Manchester Police Assistant Chief Constable Russ Jackson described the action of protestors as 'reckless and dangerous'.
Two police officers were injured, with one requiring emergency hospital treatment after sustaining a facial injury.
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The statement said:
"By late afternoon around 200 protestors had gathered outside the Lowry and over 1,000 at Old Trafford.
"Officers continued to closely monitor the situation, engaging with those present but as the groups grew in size; it became clear that many of those present were not intending to exercise their right to peaceful protest. Flares were let off and bottles thrown at officers.
"Protestors outside Old Trafford became especially aggressive and antagonistic towards police before a group of about 100 forced entry to the ground with some United staff having to lock themselves in rooms.
"Those in the stadium were evicted by officers but outside on the forecourt hostility grew with bottles and barriers being thrown at officers and horses. Two officers have been injured with one officer being attacked with a bottle and sustaining a significant slash wound to his face, requiring emergency hospital treatment.
"With the situation increasing in hostility, additional officers had to be deployed and officers had to be drawn in from neighbouring forces to assist Greater Manchester officers.
"Following discussion with The Premier League, Trafford Council and the clubs, a joint decision was made to postpone the match for safety reasons."
An investigation has been launched, the Assistant Chief Constable 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," he said.
"At different points, bottles and barriers were thrown, officers assaulted and people scaled the stadium structure creating risk for themselves and officers
"We have launched an investigation and we will be working closely alongside partners to ensure we establish the full circumstances surrounding today's events and prosecute those responsible."