Greater Manchester police (GMP) said that, of the English fans detained, seven men and a male youth were arrested on suspicion of public order offences; two men on suspicion of affray; one man on suspicion of possession of a bladed article; one man on suspicion of an offensive weapon; and one man on suspicion of criminal damage.
Of the Italians who were arrested, three men are being held on suspicion of shoplifting; three over public order offences; and one man on suspicion of possessing an offensive weapon.
GMP's match commander, Superintendent Charlie Coxon, said the incidents included "a disturbance on the forecourt that took place while police were maintaining the segregation line to allow the Roma fans to enter the stadium safely".
"Unfortunately, due to the behaviour of a very small minority of United fans, police were forced to use batons and protective equipment, horses and dogs in order to prevent the disturbance spreading," he added after the game.
"Officers made 21 arrests and received no reports of injuries.
"It's important to realise that the vast majority of the 75,000 crowd did exactly what United and GMP asked for, which was to come to Old Trafford and enjoy another exciting night of European football."
Yesterday's violence followed trouble during the first-leg of the quarter-final last week at Roma's Stadio Olimpico, with fans of the English club claiming they had been attacked by Italian policemen.
But police stressed that last night's scuffles do not come close to the extent of last Wednesday's violent scenes.
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