The main stand-off took place at the front entrance to the stadium on Matt Busby Way as riot police and dog handlers separated rival fans.

Italian fans were bombarded with bottles and cans of beer as they were subjected to weapon searches at the away end turnstiles.

Several Roma supporters criticised the delay in getting them into the ground before the vast majority of home fans arrived for the match.

The first signs of trouble were said to have been triggered by a group of about 30 Manchester United fans who attempted to charge through the police segregation line.

Eyewitnesses said a firework was fired by Italian fans at Manchester United supporters as bottles were then hurled between rival groups.

However, the vast majority of bottles were thrown from United fans as police attempted to capture the perpetrators on film by using hand-held video cameras.

Some of the United fans sang “Where are your famous ultras now?”, a reference to the hardcore Roma supporters’ group.

Greater Manchester Police said there were no reports of any injuries, but there were several walking wounded outside the stadium before the match.

Parents with young children in the Roma supporters’ queue were led tearfully to safety as the wave of missiles continued up until kick-off when the crowds finally dispersed.

Fourteen of those arrested were English and seven were Italian, police said.

Eight United fans, including one juvenile, were arrested on suspicion of committing public order offences, one was detained for allegedly possessing a bladed article, one for allegedly possessing an offensive weapon, 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 arrested on suspicion of public order offences, a third was held for allegedly possessing an offensive weapon and a fourth was held 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 10.15pm and were led largely without incident to the car parks where a fleet of coaches transported them back to the airports.

However, there were further reports of clashes involving Roma fans who were heading back on foot into the city centre.

Greater Manchester Police said Tuesday’s incidents were “small and isolated” and were resolved quickly.

Following the game, the Match Commander – Superintendent Charlie Coxon – said: “There were some incidents prior to the match including 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.

“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.”