At least five people have been stabbed and a number of others injured after "unacceptable" violence broke out in four London areas as soaring temperatures gripped the British capital.
Three people, including a police officer, were stabbed when a water fight at Hyde Park turned nasty on Tuesday, and four officers and a member of public suffered other injuries.
In Burgess Park, south London, two 16-year-old boys were knifed as disorder broke out among 500 to 600 young people taking part in an end-of-term water fight in the sun, while a nearby supermarket was ransacked.
There was further disorder well into the night when police were pelted with missiles as they moved hundreds of revellers from an illegal block party in north-east London.
And a 15-year-old girl was taken to hospital on Monday evening with suspected knife wounds to the head after hundreds gathered for a street party in St Mary's Road, Harlesden, north-west London, police said.
Two women, aged 31 and 35, were arrested on suspicion of GBH at a nearby flat and taken to a north London police station.
The violent outbursts come as Britain swelters in a heatwave, with Tuesday's temperatures soaring to 33.5C (92.3F) in Brize Norton, Oxfordshire, on the hottest day of the year so far.
Met Police Commander BJ Harrington said: "The people of London this morning are waking up to what is unacceptable criminal violence."
Speaking of the Hyde Park disorder, he said: "What we saw was a group of people spontaneously coming together to enjoy the sunshine. A group within that then decided to set up an unlicensed music event.
"Officers from Westminster borough initially went there to try and ensure that was safe and they were attacked as they tried to negotiate and talk to the organisers of this event.
"We saw five officers injured in total: one stabbed in the hand, one hit in the head with a bottle, and absolutely unacceptable levels of violence against officers, and indeed members of the public."
The Hyde Park crowd, which police said peaked at around 4,000, gathered for the water fight by the Serpentine as temperatures soared, but a "significant minority" became hostile towards officers as the evening wore on.
Scotland Yard said all five officers were taken to hospital for treatment for non life-threatening injuries and have since been discharged.
The members of the public who were stabbed remain in hospital, where their conditions are not thought to be life-threatening.
Mr Harrington denied that the disturbance, which involved a "hard core" of around 100 people, had anything to do with the Black Lives Matter movement.
The spell of hot weather was no excuse for violent criminal behaviour, he added.
He said: "Does the weather cause people to be violent? My personal opinion is no, that's not the case.
"Four thousand people assembled in a London park - that's not unusual for one of our central London parks. What is unusual is where people behave criminally, where their behaviour is unacceptable, where they attack officers, they attack each other."
Mr Harrington said he was "proud" of the police response and promised arrests would be made as investigators review a large amount of CCTV and helicopter footage.
In Burgess Park, fights between small groups broke out just before 6pm which then escalated, and a crowd of between 1,000 and 1,500 people was dispersed by 11pm.
The two 16-year-olds who were stabbed suffered non life-threatening injuries, and one remains in hospital.
At around 8pm police were called to a supermarket in Dunton Road, Southwark, to reports of 200 people causing damage inside the store and stealing, with two female customers suffering minor injuries.
Detective Chief Superintendent Simon Messinger, borough commander for Southwark, said: "While at this stage we do not believe the events at Burgess Park to be linked to disorder elsewhere in London last night, clearly there are similarities with incidents quickly breaking out amongst a large group of youths."
Elsewhere, footage appeared to show scores of people running from the scene as officers from the Metropolitan Police's Territorial Support Group interrupted an unlicensed music event on the Stamford Hill Estate in Hackney.
Some officers suffered minor injuries as the crowd threw bottles and other items, and three police vans were damaged.
Mr Harrington said police would not tolerate violent behaviour and will be looking into whether any of the incidents were linked.
Extra officers will be on duty on Wednesday night across London and throughout the weekend.
Any witnesses to the Hyde Park disorder are asked to contact the incident room on 020 8246 0076 or contact Crimestoppers anonymously on 0800 555111, or police on 101.
Mobile phone footage shared on social media showed young people vaulting over the counter of McDonald's in Marble Arch, as crowds spilled out of neighbouring Hyde Park.
Families with children were in the branch as food and drink were looted, and staff were forced to retreat to a back room to wait for police who were called at 9.15pm, the fast food chain said.
A McDonald's spokeswoman said: "I can confirm that our Marble Arch restaurant was closed for a period of time on the night of 19 July due to customers entering the kitchen area.
"Police were contacted by the restaurant team who attended and dealt with the situation, allowing us to reopen shortly after."