Many United fans complained of crushes and slow queues as they tried to enter the away section at Eastlands for the first leg of their semi-final tie at the home of their rivals. But Sportsmail understands that the delay was down to the rigorous searching of supporters known to be arriving at the turnstiles with concealed weapons.
Red hot atmosphere: United fans let off a flare during the Carling Cup semi final
Police reports and first-hand accounts revealed last night that both United and City fans threw coins, plastic bottles and even mobile phones at each other as the game began at City' s stadium. Some innocent fans, including women and youngsters, complained of cuts.
Seats in the away end were also ripped out and thrown into neighbouring sections while out on the concourse, toilet cubicles were trashed and urinals ripped from the walls.
But it is the revelation that some United fans were carrying darts and golf balls that will alarm the authorities as they look to uphold the image of the English game ahead of a bid for the 2018 World Cup and - on a local scale - ahead of next week's eagerly awaited second leg at Old Trafford.
With a flare being let off inside the away end before kick-off and fireworks being detonated outside - presumably by fans who couldn't get in - Tuesday's game had a raw feel to it. Many onlookers commented that it constituted the most electrifying Manchester derby atmosphere for some years.
Tension in the air: Police and stewards stand between rival fans
However, it appears that the evening carried a sinister edge, too, as a minority of the 9,000 away fans and 37,000 home supporters seemed intent on trouble.
Accounts from the Greater Manchester Police and from eyewitnesses revealed yesterday that there were skirmishes outside the stadium before and after the game, with rocks thrown across roads and bricks hurled through car windows. United fans also tried to break through police cordons guiding them to and from Eastlands to confront City followers, and there was one report of a man in his sixties being attacked without provocation near the stadium before kick-off.
Last night police said that 18 arrests had been made on what they were describing as a 'challenging and potentially dangerous' night. Of those arrests, six were City fans, 11 were United supporters and one was thought to be a neutral.
Next week's return game will now come under renewed focus from the police and from both clubs. With 9,000 City fans due at Old Trafford it is clear that many will arrive carrying a sense of gr ievance, either real or imaginary.
It is likely that the police will seek to remind players from both teams of their obligations once the game has started after Gary Neville, Carlos Tevez, Patrice Evra and Edwin van der Sar came under the spotlight on Tuesday.
Making a stand: United supporters hold up a banner protesting against the club's owners
Neville is being investigated by the FA after appearing to make an offensive gesture to Tevez when his former team-mate scored his first goal of the night with a first-half penalty.
Given that the Manchester United captain is already on a warning after goading Manchester City supporters at the end of the club's Barclays Premier League game last September, he can expect an FA fine and - in all likelihood - a suspension for his latest misdemeanour.
Tevez, meanwhile, made a provocative - albeit clean - gesture to the United directors after scoring his second by cupping his hands behind his ears.
United's French defender Evra, meanwhile, was struck by a cigarette lighter thrown from the crowd, something that some City fans believe was in response to a crude signal from the left back.
Goalkeeper Van der Sar was also accused by fans yesterday, this time of making a 'V-sign' behind his back as he prepared to take a goal kick. There is no video or still camera footage to corroborate these claims.
What is clear, now, is that the security operation ahead of next week's game will be huge. There was some criticism yesterday that United fans were released from the stadium at the same time as City supporters on Tuesday. It is understood that next week the two sets of fans will not be allowed to mingle.
Last night City stadium officials were relatively sanguine about what had happened during the game.
'It was lively but not the worst we have had here,' revealed a source. 'The Paris St-Germain game in the UEFA Cup last year was much worse. That night the away fans were fighting among themselves.'
Neville faces FA probe after Man United veteran's one-fingered gesture at TevezMad-chester! Tevez goads Neville as Man City seal win amid ugly scenesMan United rocked: Glazers' parent company reveals debts of £716mPep admits he won't stay at Barca for long haul as he agrees one-year dealCristiano Ronaldo convinced a goal rush is coming after Real Madrid dry spell