Police are investigating after a 15-year-old girl was injured at the EFL Cup tie between Manchester City and Liverpool.
The girl was treated for head injuries at Etihad Stadium on Thursday.
Greater Manchester Police said it was also investigating an assault on the junction of Ashton New Road and Alan Turing Way.
A man was also arrested for a racially aggravated public order offence, the force said.
A further two men were arrested on suspicion of trying to take a pyrotechnic into the ground.
Manchester City won the tie 3-2, seeing off cup holders Liverpool.
GMP's match commander, Supt Gareth Parkin, said: "We are aware of a number of objects, including coins and a smoke bomb, that were thrown during the match.
"We will be investigating the incidents and reviewing CCTV from the night.
"Clearly throwing objects in a confined area can be dangerous, and have serious and sometimes devastating consequences."
In a joint statement, Manchester City and Liverpool said the incidents were "wholly unacceptable" and "hugely disappointing".
The clubs added: "These incidents will be fully investigated, reported to the police and the individuals who are found to be responsible will be subject to the sanction process of their respective club.
"Liverpool and Manchester City are fully committed to working together to eradicate these issues from our fixtures. They have no place in football."
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