Three people were arrested when a water fight in Hyde Park involving some 1,500 people spilled out on to Oxford Street.
Police closed the busy central London shopping street to traffic on Friday evening to deal with the incident.
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 male was arrested on suspicion of causing grievous bodily harm and two for public order offences, police said.
Section 60 order
Officers estimate that about 1,500 people attended the event, which began in Hyde Park at 1300 BST. The water fight eventually petered out at about 2100 BST, police said.
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.