Hundreds of fans got into the stadium ahead of the behind-closed-doors Premier League contest, shown live on Sky Sports, causing the match to be delayed.
The match between United and Liverpool was due to begin at 4.30pm, but no new kick-off time has been finalised.
Fans also gathered outside the team's hotel - The Lowry Hotel in Manchester, where the United team are staying before the game - ahead of Sunday's match at Old Trafford.
It is understood the United squad have not left the hotel amid ongoing fan protests, while match referee Michael Oliver, who had initially been turned away from the stadium, is now inside Old Trafford.
The Premier League are still planning on the game going ahead, despite the disruption caused by the demonstrations, with the coronavirus bubble still intact as fans have not accessed the dressing rooms.
There will, however, have to be a sweep of the stadium once the situation is under control.
Many United supporters are still angry and upset over their club's role as one of the founder members of the proposed breakaway European Super League.
The plans added to years of discontent and protests from supporters following the controversial takeover by the Glazers in 2005.
United's game against Liverpool is scheduled to start at 4.30pm, live on Sky Sports. If United lose the match then Manchester City will be crowned champions with four games to go.
United owner Joel Glazer, unveiled as Super League vice-chairman when the bombshell announcement was made, apologised in an open letter to all the club's fans after they were forced to pull out of the plans.
However, that apology seems to have been rebuffed by the club's supporter base who protested outside Old Trafford last week which followed even more protesting at United's training ground, Carrington.
Ole Gunnar Solskjaer says United fans have a right to voice their anger about the European Super League and the protests have not stopped there, with 10,000 in total expected to turn up outside Old Trafford on Sunday.
Sky Sports reporter Geoff Shreeves at Old Trafford:
"Protestors have been on the pitch, they've touched goal posts so from a Covid perspective what kind of problem will that throw up? We really do not know as yet whether this game will go ahead. Security and staff are stepping in now. Before, they were heavily out-numbered, but this is a smaller group.
"The stewards are trying to contain them in that pocket, but we'll just have to wait and see how this develops.
"It's a massive security breach. Not once, but twice."
Sky Sports pundit Gary Neville gave his views on the Glazers on Sunday before protesters gained access to Old Trafford:
"The Glazer family are struggling to meet the financial requirements at this club and the fans are saying that their time is up," he told Sky Sports.
"My view is quite simply that they're going to make a fortune if they sell the club and if they were to put it up for sale now I think the time would be right, and it would be the honourable thing to do.
"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.
"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.
"If you think about the club they picked up in 2004, it had the best stadium in the country, one of the best in Europe, it had the best training ground in this country, and probably one of the best in Europe.
"It had a team that was consistently getting to Champions League quarter-finals, semi-finals and finals regularly and winning the league every season or every other season.
"If you look at the club now, this stadium I know it looks great here but if you go behind the scenes it is rusting and rotten. If you look at the training ground it's probably not even the top five in this country, they haven't got to a Champions League semi-final for 10 years.
"We haven't won a league here at Manchester United for 8 years. The land around the ground is undeveloped, dormant and derelict while every other club seems to be developing the facilities and the fan experience."