The 8-year-old girl was raped in Nuthurst Park, Moston, before the offender was chased off by other members of the public.
Officers swiftly attended and arrested a teenage boy in a nearby shop on Nuthurst Road on suspicion of attempted rape.
He remains in custody for questioning.
The victim is being provided with support from specially trained officers.
Inspector Dave Whelan of GMP’s City of Manchester team said: “I know this incident has understandably caused a lot of unrest in the local community, particularly among those who were in the nearby area at the time but I want to assure everybody that we have launched a full investigation.
“We treat all reports like this with the utmost seriousness and as such officers swiftly attended the scene and arrested a teenage boy within 16 minutes of the initial call coming in. We would like to thank the local community who assisted in our initial enquiries earlier today.
“Local residents may notice an increased police presence in the area so if you have any information about the incident or concerns that you wish to raise with officers, then I would encourage you to either approach them directly or call police.”