Police were called to Manchester United's training ground on Friday after a group of supporters arrived to protest against the club's owners, the Glazer family.
United were forced to lock the front gates at Carrington and divert visitors to a back entrance after a group of around 30 fans carrying "Glazer Out" banners gathered outside the complex at around midday.
It comes a year after protesters succeeded in breaking into Carrington before speaking briefly to former manager Ole Gunnar Solskjaer.
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There is another protest planned at Old Trafford ahead of the game against Norwich City on Saturday with supporters planning to only enter the stadium until 17 minutes after kickoff, one minute for each year of Glazer ownership.
"We all know football is a game of passion and emotions, we can all understand, I can understand our supporters are disappointed with our standing in the table," interim manager Ralf Rangnick told a news conference on Friday.
"But I still believe our supporters are one of the best, if not the best, in England and as long as they do that protest in a peaceful way and support the team in the stadium, they have a right to express their opinion. I can understand they are disappointed."
Sources have told ESPN that United are on the brink of announcing Ajax manager Erik ten Hag as their next permanent boss but Rangnick refused to be drawn on the club's plans.
"I don't know if it's a done deal," Rangnick added. "My focus is on the game against Norwich; to prepare the team on a higher level for the game.
"We all know he's a good manager and he's done a good job at Ajax and former clubs but it doesn't make sense if I tell you what I think of another coach."
Rangnick is preparing to face bottom club Norwich without Luke Shaw, Raphael Varane, Edinson Cavani, Scott McTominay and Fred -- all of whom are injured.
Shaw has undergone an operation to remove metal screws from his leg after suffering a severe fracture in 2015 and Rangnick is not confident the left-back will play again this season.
"He will be out for at least another four or five weeks," Rangnick said. "It will be difficult for him to be available for the remaining seven games."