Eurosport - Thu, 28 Jan 14:15:00 2010
The incident happened as Bellamy retrieved the ball for a corner. First a coin struck him on the back of the head then other missiles, including several bottles, were thrown as he went to ground.
Manchester Police are scrutinising CCTV footage to try and identify the thrower of the coin, while United face potential FA disciplinary action over their fans' actions - which may range from a fine to being forced to play matches behind closed doors.
A spokesperson for the club said: "Our security thinks they [the perpetrators] were identified and arrested."
If caught, they face a life ban.
It soured a thrilling 3-1 win for United that took them into the Carling Cup final, where they will play Aston Villa.
Police arrested 11 people before an encounter made volatile by inter-city rivalry and the return of City striker Carlos Tevez to the home of his former employers, while 29 were arrested before and after the first leg at Eastlands last week.