The unprecedented cancellation of a game in the world's richest soccer league due to fan protests is the culmination of long-running anger against the American owners that began with a 2005 leveraged takeover that loaded debt onto the club.
But the wrath of supporters has boiled over in the two weeks since the club was part of the failed European Super League breakaway that collapsed amid a groundswell of condemnation.
Supporters gaining access to the pitch - before the two teams even travelled to Old Trafford - was even more startling given the strict coronavirus measures in place around the stadium that keep games closed to any spectators.
One of the flares set off by fans was launched into the stands where the Sky Sports television team was already broadcasting more than two hours ahead of the scheduled kick-off.
Although the protest had been announced in advance, the stadium forecourt on Sir Matt Busby Way was still able to be accessed by thousands of fans - many chanting "We want Glazers out" as flares were set off.
The crowd was only dispersed after baton-wielding police and officers on horseback charged fans as clashes erupted under a shower of flying glass bottles about 20 minutes before the game was due to start.
Two police officers were injured and one required emergency treatment for a "significant slash wound to his face" after being attacked with a bottle, the Manchester force said.
United said the game was postponed "due to safety and security considerations around the protest" after discussions with police, authorities and the league.
The Premier League, which has yet to announce a new date for the match, expressed concern about the disorder.
Protesters had also gathered outside the United team hotel in central Manchester.
"The security and safety of everyone at Old Trafford remains of paramount importance," the Premier League said in a statement.
"We understand and respect the strength of feeling but condemn all acts of violence, criminal damage and trespass, especially given the associated COVID-19 breaches.
United had no debt until the Glazers arrived and the latest half-year accounts showed it had soared 16% year-on-year to 455.5 million Pounds ($A815.5 million).
The Glazers, who also own the NFL's Tampa Bay Buccaneers, have declined to engage with fans or media since buying United in 2005.
Former Manchester United defender Gary Neville, who was inside Old Trafford preparing to commentate on the game for Sky Sports, called on the Glazers to put the club up for sale.
"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," Neville said.
If United had lost the planned game against Liverpool, neighbours Manchester City would have won the Premier League title for the third time in four seasons.