The force said eight children had been taken to hospitals with "potentially serious injuries".
In a statement, police said: "We've evacuated Woking Park this evening, where people were gathering for the fireworks celebrations, following the collapse of the Helter Skelter at the fairground".
Eight children were seriously hurt after an accident on a giant fairground inflatable slide at a fireworks display on Saturday.
The eight children injured have been taken to major trauma centres, although there is no update on their conditions, South East Coast Ambulance Service said.
The 23-year-old, from Guildford, Surrey, said he saw up to 40 children playing on the attraction.
The children were rushed to major trauma centres.
The husband, who did not wish to be named, said: "My wife and family were walking towards the slide at around 7.20pm and as we were approaching we saw the side [of the slide] began to buckle about half way or three-quarters of the way up. It looked pretty flimsy to say the least", he said.
"There were far too many kids on it".
Organised by the Rotary Club of Woking District, the "Fireworks Extravaganza"' is considered to be one of the best firework displays in the South East, and in the past has been one of the most popular events held in Woking.
She said: "I am very grateful my children didn't go on it, let's just put it that way".
He said he watched the fireworks with friends before seeing "flashing lights".
The fireworks display was abandoned shortly after the children were injured.
He said that, although it was "too early to speculate" on the cause, the organisation was co-operating with police.
Witnesses claim that 5 kids were messing around and fell off the top.
One person said: "Was at the horrific incident at #WokingPark tonight".
Local musician Anna Neale said she had planned to take her daughters, aged 10 and six, on the giant slide after the fireworks.
"We are all very shocked and distressed by events in the funfair this evening".
Woking Fireworks tweeted: "Really sorry but due to a major incident in the funfair area the police have asked us to evacuate the park".
The 36-year-old mother-of-two said: "We saw all the blue lights and saw the air ambulance arriving". We have had to evacuate the whole Woking Park site to allow the air ambulance to land. "Everyone was just shocked".
A Health and Safety Executive spokeswoman said "initial enquiries" were being made, while Woking borough council said: "We will be working with the police and relevant authorities to understand and explain what happened". We are focusing in assisting the emergency services in dealing with the injured children.
"I hope everyone is ok".