Nine men were arrested after police were called to a water fight in Hyde Park said to involve up to 250 people.
One man, in his early 20s, was shot by police with a Taser gun after he set his pit bull-type dog on an officer, during the disturbances.
The event had been organised through a chatroom but deteriorated amid reports of knives and injuries to three children unseated from their horses.
Police using dogs sealed off the area at 1930 BST and arrested nine men.
Four of them were arrested for allegedly assaulting officers.
'High spirited'
A spokesman said all the suspects were being held at central London police stations.
Officers described the actions of those involved in the water fight as being "high spirited".
But during the incident horses in the park became frightened and three children, aged between five and 12, were unseated in Serpentine Road.
They suffered injuries including suspected broken bones and concussion.
Reports then came into police that some people involved had been seen with knives.
A police spokesman said officers from the territorial support group, dog handlers and the British Transport Police arrived to assist Royal Parks officers.
Those arrested include four on suspicion of assault, three under the Public Order Act, one for affray and one person accused of handling stolen goods.
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