The violence at Manchester United Sunday was "completely unacceptable" and should be condemned in "the strongest possible terms," police leaders said Monday.

Meanwhile, the Football Association told Sky Sports News it is looking into the events that led to the game's postponement and it is talking with the club, the Premier League and safety authorities.

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 said.

The violence happened during protests before United's scheduled game against Liverpool, which eventually had to be postponed for safety reasons.

Manchester police superintendent JP Ruffle said the protest was an "absolute disgrace, disgusting scenes."

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.

"Passions are running high in football but there are ways to protest and make your voice heard without hurting or endangering others," Nigel Huddleston, Minister for Tourism and Sport, said according to Reuters.

"We understand the frustrations, but the violence by a small minority of fans at Old Trafford yesterday was unacceptable," he said.

The fan protests against the Glazers, which have taken place since they bought the club in 2005, have been reignited since United's involvement in the attempt to create a breakaway European Super League.

James Cleverly, a Foreign Office minister, was speaking after Manchester United's game with Liverpool was postponed after a peaceful planned protest outside the stadium by fans angry with the club's owners turned violent.

"Looking at the scenes that we have just seen on the program, there can't be an excuse for that kind of behavior," Cleverly told Sky News, adding that supporters had to be at the heart of the game and referring to a "fan-led" review announced by the government.

"So 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."

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