A 20-year-old man has died of a suspected drug overdose and three others were stabbed, as thousands attended two raves in Manchester on Saturday night.
The man had attended a gathering of around 4,000 in Droylsden, according to Greater Manchester Police.
Nearby in Carrington there were reports of three separate stabbings at another 2,000-strong party, including one which left an 18-year-old man with life-threatening injuries.
Two other men, aged 25 and 26, were also injured.
Police said they are also investigating the rape of an 18-year-old woman who attended the Carrington event.
They added that officers were met with violence from revellers at the party, with items thrown and a police car being vandalised.
There were no further reported incidents of crime at the Droylsden rave, which took place in Daisy Nook Country Park.
Footage posted on Snapchat shows people densely packed around a stage which read Quarantine Rave at the party, well within two-metre social distancing rules, and on Sunday morning the park was littered with an "atrocious" amount of rubbish according to volunteers who cleaned it.
Our officers are working incredibly hard to keep people safe and should never be met with inexcusable violence whilst carrying out their duties
“These raves were illegal and I condemn them taking place – they were clearly a breach of coronavirus legislation and guidelines, and have had tragic consequences,” said Greater Manchester Police Assistant Chief Constable Chris Sykes.
“Coronavirus is still a threat and we will continue to engage with people to encourage them to take some personal responsibility and do the right thing.
“Ultimately we need the people of Greater Manchester to join together and demonstrate the spirit that they have previously shown in the face of hard times.”
Mr Sykes added that violence towards police at the Carrington rave was “absolutely not acceptable and will not be tolerated”.
“Our officers are working incredibly hard to keep people safe and should never be met with inexcusable violence whilst carrying out their duties,” he added.
Around 70 volunteers helped to clear the laughing gas canisters, balloons and bags left in Daisy Nook Park on Sunday morning, including Warwick University student Georgia Blakeney.
"The mess was atrocious," the 21-year-old from Ashton-under-Lyne said.
"The people attending were my age group, I'm a uni student who can't wait to go out with my friends again but I can't believe people were so selfish to do it in such a way.
"I think it's an insult to not only front line NHS workers and high-risk individuals but those who are suffering from the lockdown from things like economic damage and poor mental health, which will no doubt be extended if things like this happen.
"The people who attended today, risking their own lives reinstate a bit of hope into society - they were brilliant."
Deputy Labour Leader and MP for Ashton-under-Lyne Angela Rayner tweeted: “Those who attended put themselves and their loved ones at risk. (Completely) irresponsible.
“A lot of effort went into online events this weekend across (Greater Manchester). Those who attended should be ashamed of themselves. My local area now has to deal with the aftermath.”