It came after many more blockaded the entrance to the Lowry Hotel where the United players and staff are staying.
It is understood Sunday afternoon’s match against Liverpool will still go ahead at 4.30pm as scheduled although the teams have yet to arrive at Old Trafford.
Liverpool have been held back at their team hotel by Greater Manchester Police.
Fans shouted ‘we want Glazers out’ and ‘United, United’ as they walked onto the pitch before vacating the stadium a few minutes later after making their feelings known.
Thousands had assembled at an organised protest outside Old Trafford against the Glazer family and their efforts to sign United up to the European Super League.
Hundreds of Manchester United fans have stormed the pitch at Old Trafford in protest at the club’s American owners ahead of Sunday afternoon’s match with Liverpool
These were the extraordinary scenes inside the stadium as protesters breached security to get onto the turf just two hours ahead of the Liverpool game
Protesting fans set off smoke flares outside Old Trafford as they make their feelings known
The Glazer family, United’s owners, signed the club up to the European Super League before making a u-turn 48 hours later amid a furious backlash
Thousands of United fans protested outside and inside Old Trafford against the Glazer regime
Some demanded the installation of Germany’s 50+1 ownership model at the protest
Security personnel battled in vain to keep the protesters outside the stadium
And a section of the crowd managed to storm the entrances to the stadium, breach security and get onto the pitch.
The departure of the team bus on the short journey from central Manchester to Old Trafford was delayed as police tried to force back the crowds and restore order.
Smoke bombs were set off both outside the Lowry and outside Old Trafford.
United were one of six Premier League clubs who initially signed up to the doomed project last month. They all withdrew 48 hours later following a furious backlash from the wider football community.
Joel Glazer made an apology to supporters for signing up to the doomed Super League project but many United fans carried placards at the protest reading ‘apology not accepted’.
Angry fans make their feeling known outside Old Trafford as part of Sunday’s protests
Fans told the glazer family their apology over joining the Super League wasn’t accepted
Fans in green and gold protest scarves chant outside Old Trafford on Sunday afternoon
Stewards struggled to contain the fans who invaded the pitch through a corner of the stadium near the Stretford End.
They kicked footballs laid out for the pre-match warm-ups. Many of the fans wore green and gold scarves – the colours of Newton Heath, the predecessor of Man United – which have become emblematic of the anti-Glazer movement.
One supporter was filmed picking up a camera tripod from a pitchside media position and hurling it onto the pitch. Others tried to get down the tunnel, while another took a corner flag.
But the crowd did quickly disperse back outside the stadium after a few minutes.
More to follow