AJ Tracey's manager was fined £10,000 after the star rapper's impromptu appearance at Platt Fields Park in Fallowfield, police have revealed.
And GMP said they expect the penalty notice for breaching coronavirus legislation to be paid in full.
The force has spoken out to reveal what decisions were taken and why after hundreds of people responded to a message the 27-year-old hip hop star posted on his Twitter page with more than half a million followers.
Superintendent Graeme Openshaw said it was 'astonishing' Tracey's team didn't foresee a public appearance would create large crowds.
He said the rapper's entourage was asked to leave.
But as crowds of hundreds were already present, Supt Openshaw said police decided 'to manage what has already happened and allow the act to do a very short appearance'.
"It is a very difficult balancing act," he said.
"We do not want police action to be engaged with a peaceful crowd. The decision we took was that we would try and manage things as safely as possible."
The BRIT Award 2021-nominated west London rapper wrote on Twitter on Sunday: "Next stop manny!
Get the latest updates from across Greater Manchester direct to your inbox with the free MEN newsletter
You can sign up very simply by following the instructions here
"Come link me Platt Fields Car Park 4pm."
Police only knew about the event from reports of crowds gathering.
Police vehicles were shown parked up beside a van with 'AJ Tracey' livery on it and officers were seen in conversation with Tracey's team.
One witness reported a 'stand off' situation over the event.
Crowds then began to gather on the basketball courts inside the park, interspersed with police officers, before Tracey arrived in a blacked out van flanked by officers.
Police were stood around the courts as the rapper met with fans inside. Many hung over railings and sang.
Tracey was also photographed posing for pictures with fans.
In a Twitter video posted later and titled 'sorry guys', the rapper said he didn't expect that many people to turn up and was going home to London.
Supt Openshaw called the situation 'extremely disappointing'.
"He is a large and popular act and he posted on social media saying he was coming to Greater Manchester," he told the Manchester Evening News.
"He expressed a wish to meet people.
"His management company had not had the foresight to engage with us or the local authority.
"When we first became aware, which was in the middle of the afternoon, we responded to the park. There was already a large crowd there.
"We have engaged with the crowd to try and get them to move away and with the act's management team. That has failed."
Supt Openshaw said the thoughts of the police then turned to managing the crowd safely away.
"That is quite difficult with crowds," he said.
"You have to judge the dynamic. We took the view that we would rather try to manage what has already happened and allow the act to do a very short appearance.
"There was no engagement at all between the artist and their management with the police and the local authority beforehand. We had, in effect, an unlicensed gathering."
Supt Openshaw said AJ Tracey's team were directed to leave.
They told police they wanted to stage a 'meet and greet' and distribute CDs, he said.
"I do not think they understood the consequences of what they were doing," Supt Openshaw said. "I do not think they thought about the full circumstances.
"It is incredibly disappointing. He is a very, very popular music act. We would expect his management to be switched on to his popularity.
"How they have not foreseen that it would not create a crowd is astonishing."
Supt Openshaw said AJ Tracey's manager 'put himself forward as the event organiser' and that he expected the £10,000 fine to be paid.
"We have been here several times over the last year or so, in trying to manage some unique situations," he said.
"We need to be careful about how we execute our powers. There is no black and white position.
"We try to establish principles and stick to them."
Police, meanwhile, have praised the public for behaving responsibly over the first weekend since pubs and restaurants were allowed to open with outdoor table service as Covid lockdown restrictions began to ease.
Supt Openshaw said police had worked closely with licensees and there was 'very little incident'.
The force said 25 fixed penalty notices were issued from Friday to Sunday for Covid breaches - a huge fall on numbers over past weekends.