The supporters gained access to the training ground with banners reading "Glazers out" and "we decide when you play".
It was reported the group comprising of around 20 people blocked entrances as players arrived for training. They had a conversation with United boss Ole Gunnar Solskjaer before leaving the training ground.
A United statement read: "At approximately 9am this morning a group gained access to the club training ground.
"The manager and others spoke to them. Buildings were secure and the group has now left the site."
United were one of six Premier League clubs and 12 teams overall to sign up for the controversial competition, which drew widespread condemnation from fans, players and pundits alike.