Roma fans have been involved in violent battles with police outside Old Trafford.
Missiles were thrown and officers drew their batons to control the thugs.
Up to 100 Greater Manchester Police officers tackled groups of Italian fans outside the Manchester United stadium.
Sky Correspondent Tom Parmenter said: "This was instigated by the fans who have come over from Italy but certainly there were United fans involved."
He said the scenes were "frightening scenes for about 10 minutes".
Witnesses spoke of gangs of hooded fans dressed in black making their way to the front of the ground before the trouble flared.
There had been fears Roma hooligans, known as Ultras, would provoke trouble.
It follows violence in Rome when several United supporters were stabbed on the way to the Olympic Stadium.
Inside the ground, riot police were accused of battering visiting fans.
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