Rapper AJ Tracey said he 'didn't expect that many people to turn up' after huge crowds flocked to see him at Platt Fields Park in Fallowfield.
Greater Manchester Police have now confirmed a £10,000 fixed penalty notice has been issued to the organiser for breaching coronavirus legislation.
The mayor of Greater Manchester, Andy Burnham, had already said on Twitter afterwards that the 'organisers have been fined'.
Organisers of illegal large-scale gatherings can be hit with £10,000 fines under coronavirus legislation.
In a statement to the Manchester Evening News on Sunday evening, GMP said officers were called to the scene of a 'large gathering' at 2.50pm on Sunday and enquiries established a 'music event' was being held.
The force said: "Officers attended and attempted to engage with individuals and ensure the safety of those present. People have now dispersed and the organiser of this gathering has been issued with a £10,000 Fixed Penalty Notice.
"GMP would like remind the public of the importance of adhering to the lockdown regulations in order to keep our community safe and protected.
Video footage and photographs from the scene showed hundreds of people gathered around basketball courts inside the park off Wilmslow Road.
The BRIT Award 2021-nominated west London rapper wrote on Twitter on Sunday: "Next stop manny!
"Come link me Platt Fields Car Park 4pm."
It's understood the rapper was in Birmingham earlier - and had planned to visit Bristol later on Sunday evening.
One witness at the scene in Manchester said Tracey, real name Che Wolton Grant, had promoted an impromptu live gig.
Greater Manchester Police vehicles were shown parked up beside a van with 'AJ Tracey' livery on it as crowds began to gather around a car park.
Officers could be seen in conversation with people stood near to the van - and one witness reported a 'stand off' situation over the gig.
Crowds then began to gather on the courts inside the park, interspersed with police officers.
Tracey arrived at the scene in a blacked out van flanked by officers.
Police were stood around the basketball courts as the rapper met with vans inside as they hung over railings and sang.
Tracey was also photographed posing for pictures with fans.
A witness at the scene said: "There was some filming and singing and crowd interaction - and that was about it."
He left a short while later and the crowds dispersed.
Manchester city council Covid marshals were also pictured at the scene with crowds.
In a video posted to his official Twitter page after the event at Platt Fields Park, Tracey thanked fans in Manchester for 'turning out'.
But he said: "It's not going to be safe for me to come to Bristol.
"I didn't expect that many people to turn up in Manny [Manchester] and genuinely...it's not okay for me to go ahead.
"I'm going to head home to London.
"I appreciate you lot showing support and when it's safe to do so I am definitely going to come back and do some shows."
Superintendent Caroline Hemingway, of Greater Manchester Police, said: "I would like to remind the public that the recent easing of restrictions does not mean the pandemic is over.
"We are still very much in the midst of a public health crisis and it remains as important as ever to abide by the COVID-19 legislation.
"With the roadmap in place, there is finally hope that we will soon be returning to a sense of normality, which is why it is so important not to flout the lifting of restrictions and risk spreading the virus, as this could lead to delays in further restrictions being eased.
"I would like to thank the majority of the public who continue to following the legislation. This has been an incredibly difficult year for everyone and your commitment in helping to protect the most vulnerable members of our society has not gone unnoticed.
"People can now meet in a group of six or as two households.
"Please continue to abide by these rules and help to keep our community safe."
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