Officers said they believed he had been dead for a “matter of days”.
The search of the area has not yet uncovered anything besides the torso and there were no identifying marks on the body part.
Chief Superintendent Tony Creely, district commander for Salford, said: “At this time we continue to work hard to determine the identity of the person whose remains have been found.
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“Through initial forensic and DNA testing, the victim is a man who is likely to be aged over 40 and who had only been deceased for a matter of days.
“We have a team of specially trained detectives within the major incident team working around the clock to investigate the full circumstances that led to the victim being found here.
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“Last night we made an arrest of a man in his 20s from the local area on suspicion of murder.
“He remains in custody and will be questioned by detectives later today.”
CS Creely had earlier said there were “a number of unknowns around this case”.
He continued: “At the heart of our investigation is this man and his family. We are using all forensic techniques available to identify him as soon as we can so we are able to support his loved ones during this devastating time.”
The investigation “will take time”, he added, but police are working to “provide answers as soon as we are able”.
An extensive police cordon has been in place at the largely wooded reserve, which sits north of Manchester city centre and covers around 32 hectares of countryside, according to its website.
Specialists from the police forensics units, search teams, and a dog unit have been working at the site, police said.
Disruption of the local area may continue for some time, police said.
The force urged anyone who may have seen anything suspicious in the Kersal Dale area recently to contact them.