Five football fans are in custody after Manchester United's defeat to bitter rivals Liverpool descended into violence.
Flares were set off by travelling fans before a group of Liverpool supporters in the home section came under attack as tempers frayed at the end of Thursday night's crunch match.
Punches were thrown before stewards and police intervened to protect the away fans as vile taunts were exchanged between both sets of supporters during the last-16 second leg tie at Old Trafford.
Superintendent Jim Liggett, match commander for the game, said there were a "handful" of "concerning incidents" at the end of the game which are under investigation.
He added: "We have made a total of five arrests; however, the behaviour of the majority of fans in attendance was excellent and I would like to thank them for their co-operation throughout."
Earlier, fans travelling to Manchester from Merseyside for the game were greeted with a banner saying "Murderers" and a reference to the Hillsborough disaster strung over a motorway on the way to Old Trafford. Police quickly removed the banner.
At the game, Liverpool fans celebrated dumping their rivals out of the competition on home turf by lighting red flares. Then scuffles broke out in the top tier of the East Stand when a handful of away fans, who were sitting among home supporters, unfurled a club flag - much to the delight of the visiting fans below them.
Greater Manchester Police said two men were arrested on suspicion of affray, one man on suspicion of causing criminal damage after the window of a minibus was smashed, another for being drunk and disorderly and another was given a caution for lighting a flare.
The fixture has seen bad blood between the two clubs for years and during last week's first leg at Anfield, won 2-0 by the hosts, a number of offensive chants in reference to the Hillsborough tragedy were sung by a large section of the away end.
Midway through the second half at Old Trafford there were two very faint separate chants of "Murderers" from the Stretford End but both died out almost before they had got started.
Last week Manchester United condemned the chants but Uefa took no disciplinary action as no reference was made in the Spanish match officials' reports.
"It has always been the position of Manchester United that chants of this nature, which refer to historical tragedies, have no place in the game and do not reflect the values the club holds," said United's statement on Friday.
"We are in discussion with our fans' groups to seek their support in preventing this type of behaviour in the future."
Greater Manchester Police were alerted to the motorway banner and swiftly took it down.
"Police made aware of offensive banner that was hanging over the M602," GMP said on their Twitter account.
"Officers immediately dispatched and banner now taken down."
Inside the Old Trafford stadium before kick-off one banner in the away end bearing the words "Hang the Roma 4" - in reference to United supporters convicted in 2007 of fighting with police in the Italian capital who subsequently had their jail sentences cut on appeal - was also quickly removed.
The match finished 1-1 with Liverpool going through 3-1 on aggregate.