A police officer was among three people who were stabbed when a water fight in central London turned ugly.
The large crowd gathered for the spontaneous water fight in Hyde Park as temperatures soared on Tuesday, but became hostile towards officers as the evening wore on.
Police who had been monitoring the crowds near the historic Serpentine lake from 3pm were pelted with missiles including bottles, one of which injured an officer.
Scotland Yard said both officers were taken to hospital for treatment for their injuries, although they were not life-threatening.
Two people who were not identified by police were stabbed when violence flared at around 8.40pm. Police said both were taken to hospital where one received treatment for non-life threatening injuries. The condition of the second victim is not known.
Police units moved in to disperse the crowds and the operation lasted into the early hours of Wednesday morning.
Scotland Yard said no arrests had been made and appealed for witnesses with information to call police in Westminster via 101.
To give information anonymously they can contact Crimestoppers on 0800 555 111 or at crimestoppers-uk.org.