The clashes in Hyde Park were triggered by police, called to the Serpentine lake to break up crowds on one of the year’s hottest days.
Videos on YouTube showed young people attacking vans, taunting police and firing water pistols as the fun turned into heated fury.
The mass gathering which took place on 11 June followed invitations on the internet site Facebook for a wholesale but good-natured soaking.
Police, who used on-the-spot powers to stop and search, arrested six people. The chaos echoed a similar collapse of order in the park two years ago leading to nine arrests.
A spokesperson for the Royal Parks told Horticulture Week: We encourage people to be responsible towards other park visitors and consider antisocial behaviour unacceptable.
"Public order is the responsibility of the Metropolitan Police. We are working with them to ensure the parks provide a safe and enjoyable environment at this popular time of year."