Last month, the Crown Prosecution Service (CPS) said it would not pursue a third trial against Amaaz and his brother, Muhammad Amaad, 26, who faced a single count of assaulting PC Marsden, and they were formally acquitted by the judge.
The brothers had claimed they acted in lawful self-defence, or in defence of the other, during the brawl.
In December 2024 the CPS announced no police officer would face criminal charges over the incident, which led to Greater Manchester Police (GMP) lifting the suspension of Marsden from all duties.
GMP chief constable Sir Stephen Watson said the force would fully co-operate with the IOPC investigation.
“Our officers were responding quickly to precisely the sort of outrageous criminal behaviour that rightly offends the public,” he said.
“In undertaking their duties, officers were met with resistance and violence; followed by online vilification, condemnation and adverse commentary from those who did not have the full facts.”
He said 35 GMP officers are assaulted “every week” and added: “Such incidents can never be justified.”