More than 35,000 supporters from each club will travel for a game to be played on the weekend of April 16-17 and there will be an extra headache for police if the game is on the Sunday - which it will be if City reach the Europa League quarter-finals and play the Thursday before - because it will be the same day as the London Marathon and Arsenal's Barclays Premier League clash with Liverpool.
The operation will involve Scotland Yard, Greater Manchester Police and British Transport Police. A Scotland Yard source said: 'From a security point of view, this is a complete nightmare. It will cost millions of pounds.' A Met Police spokesman added: 'We deal with football matches - FA Cup semi-finals and final, internationals - all the time. We obviously have a lot of experience of dealing with these things and an appropriate and flexible policing policy will be in place.'
It will be the first time the two Manchester clubs have faced each other in an FA Cup semi-final since 1926. Stoke take on Bolton in the other semi-final.
Meanwhile, Shaun Wright-Phillips has admitted it was inevitable City would end up being paired with their neighbours in their first FA Cup semi-final for 30 years.
Micah Richards' second-half header proved enough to give City a narrow win over Reading at Eastlands, with the Blues knowing before kick-off what the prize for victory was going to be.
And Wright-Phillips was so certain of the outcome, he hardly bothered to take any notice. The England international said: 'We found out in the warm-up and it gave us all a laugh because we had all guessed it would happen.
'A few of the lads felt it would have been nice to get them in the final, but we have them in the semi so now we have to cross that bridge when we come to it.'
Manager Roberto Mancini insists there is no element of revenge, even though there could be nothing sweeter for City fans to beat the side who hang a banner from their stadium emphasising how much time has passed since their neighbours' last trophy 35 years ago.
Even in Mancini's short time at Eastlands United have recorded four narrow wins over City, including a Carling Cup semi-final triumph last term and their latest meeting in February, which was settled by Wayne Rooney's wonder goal.
'Revenge is the wrong word,' said Mancini. Every year in football you can meet your opponent, two times minimum, maybe three or four.
'It is a great semi-final and you could see what it meant to the fans at the end. We are very close with all the top squads but we need to improve more and maybe next year we will be at the same level. But it is important that we played a good game at Old Trafford last time and we have a chance to win, like them.'