Three people are being questioned a water fight involving some 1,500 people that closed London's Oxford Street.
The water fight began in Hyde Park at about 1300 BST on Friday and was still raging eight hours later.
Police closed the central London shopping street to traffic after the incident spilled over. It finally petered out at about 2100 BST.
One male is being questioned on suspicion of causing grievous bodily harm and two for public order offences.
One person needed hospital treatment for a facial injury sustained at the event, which is thought to have been organised on a social networking site.
One posting advertising the event said: "As soon as the clock strikes 1pm, let battle commence - get as many other people wet without getting wet yourself! Last one with a dry T-shirt wins!!!"
A police spokesman said most of the people who took part in the water fight - held on one of the hottest days of the year so far - were well-behaved.
However, Scotland Yard issued a Section 60 order allowing officers to search revellers in the area.