Police and football fans were involved in a series of scuffles before the Champions League match between Manchester United and Roma on Tuesday.
The flare-up was soon contained by Greater Manchester Police
Greater Manchester Police officers made 21 arrests after confrontations around Old Trafford prior to kick-off.
The worst incident involved several dozen mainly Roma fans but it was soon brought under control by officers with batons and police horses and dogs.
Manchester United won the match 7-1 to reach the Champions League semi-finals.
Hundreds of supporters were caught up in ugly scenes outside Old Trafford before the game.
Eyewitnesses said a firework was fired by Italian fans at Manchester United supporters as bottles were hurled between rival groups.
Police said 14 of those arrested were English and seven were Italian.
Eight Manchester United fans, including one juvenile, were arrested on suspicion of committing public order offences, one was detained for allegedly possessing a bladed article, two were held on suspicion of causing affray, one for suspected criminal damage and one was arrested for a suspected breach of the peace.
Two Italian supporters were held on suspicion of public order offences, another for allegedly possessing an offensive weapon, and a fourth for allegedly possessing an offensive weapon and committing a public order offence.
Three others were arrested on suspicion of shoplifting, believed to be from the Manchester United Megastore.
Roma supporters began trailing out of the ground at about 2215 BST and were led without incident to a facing car park where a fleet of coaches will take them back to the airport.
There were no reports of any injuries.
BBC production staff witnessed Manchester United fans throwing cans and glass bottles while police with dogs tried to separate the two sets of supporters.
Police dog units and mounted officers became involved with a series of scuffles with supporters close to the club shop on Sir Matt Busby Way at about 1750 BST.
Police sources told the BBC that the "isolated" trouble - which involved fans of both teams - was contained within five minutes.
A statement released by Greater Manchester Police said: "Officers patrolling at Old Trafford ahead of the Manchester United versus Roma match have been required to deal with a number of small and isolated incidents.
"The incidents are being dealt with as they happen and are being resolved quickly.
"Some arrests were made in order to maintain the peace at the ground."
BBC radio reporter James Alexander said extra security was very much in evidence outside the stadium.
"There were lots of security personnel and extra police cars and vans around the stadium," he said.
"It was classified as a Category C match which is the highest alert there is for a match at Old Trafford. It is on a par with a home match against Liverpool."
Minister for Sport Richard Caborn said: "It's unfortunate that there have been some scuffles outside Old Trafford.
"This was always going to be a highly charged game following the incident last week. Greater Manchester Police have used proportionate force in a potentially difficult situation and have handled it well."
The visiting fans were in the second tier of the East Stand and there were no reports of trouble inside the stadium.
Roma's managing director Rosella Sensi earlier insisted their fans would behave impeccably.
Uefa is to investigate the trouble from the first leg, which could lead to disciplinary action for both United and Roma.