Pictured: The moment a man punched a girl off her feet during a Facebook water fight

Nine people were arrested for assaulting police and affray at the event, which attracted 250 youngsters to Kensington Gardens, close to Princess Diana's former home.

It was intended as a fun way to cool down in the blistering heat, but tempers flared after the girl doused a young man with a fizzy drink.

After covering his T-shirt with the red liquid she ran away laughing, but her victim failed to see the funny side. 

He chased her and punched her in the face, knocking her off her feet and leaving her helpless on the ground.

The attack happened at around lunchtime yesterday near the Serpentine in London's Hyde Park.

Shocking: A girl is attacked during the water fight in Hyde Park, central London

Gangs of youths then ran amok, threatening the public, flashing knives and causing riding school children as young as five to be thrown from their horses.

Police feared others were carrying knives after park users reported seeing men "flashing" their weapons.

One man, aged in his early 20s, was shot by police with an electric stun gun after he set his pitbull-type dog on an officer.

He and eight others were arrested for offences including attacking police, public disorder, affray and handling stolen goods.

Three children, aged between five and 12, who were riding past the brawl, were thrown when their horses became frightened. They suffered broken bones.

Paramedics took all three to hospital for injuries including suspected broken bones and concussion.

Appalled: Fellow water fighters rush to the victim's aid as the man stands over her

The terrifying scenes came two weeks after teenager Frederick Moody-Boateng was murdered having attended a similar water pistol fight in Holland Park.

About 300 teenagers had responded to the Facebook posting.

One man, who captured the events on camera, said the group had earlier been spoken to by police for disturbing people in the park.

He said: ‘About 10 minutes after the police left, the girl poured drink over this bloke. He was not happy and chased after her. She thought it was a game but he looked really angry.

‘When he caught up to her he just whacked her so hard - she was lifted completely off her feet. Hundreds of people saw it. It was shocking. He then just stood over her, threatening her until other people in the group pulled him away.’

Another witness said: 'This burly guy chased her and threw a punch square on to her face. It knocked her off her feet.

'There was a girl there who seemed to be the organiser of the event and she was in tears.'

The girl later staggered to her feet and was helped away by friends.

Play fight: The girl splashes the man with coloured liquid from a bottle

Game turns nasty: The man begins chasing his victim

Other witnesses claimed hundreds of police, some with dogs, later descended on the park to disperse the youths.

Paramedics were called to Serpentine Road to treat some of the injured horseriders for suspected broken bones and concussion.

At 7.30pm a search cordon was put in place in Hyde Park and Kensington Gardens.

A spokesman for Scotland Yard said several people became ‘hostile to police actions’, resulting in nine being arrested: four for assault on police, three for public order offences, one for affray and one for handling stolen goods.

All were men in their twenties and were taken to central London police stations.

Police are still trying to trace the thug responsible for the attack.

The incident happened on Wednesday, the same day Ministry of Justice figures revealed soaring levels of violence committed by under 18s.