The protests started at the team hotel with around 1,000 supporters outside the Lowry Hotel where the United team were staying.
Sources have told ESPN there are concerns some fans may look to disrupt the game, which kicks off at 11.30 a.m. ET.
Two police vans were positioned at the gates used by the team coaches from early on Sunday morning.
Fans started gathering on the forecourt at Old Trafford from around 8 a.m. ET. At around 9 a.m. ET, a large group broke the fencing around the stadium and moved the protest towards the Munich Tunnel underneath the Sir Bobby Charlton Stand.