Seven of the eight children who were hurt after an accident on a giant fairground inflatable slide at a fireworks display on Saturday have been discharged from hospital, police have said.

Witnesses reportedly said a number of children fell from the top of the slide on to concrete and people several metres below.

The fireworks display at Woking Park in Surrey was cancelled and a crowd of 12,000 people were evacuated from the venue to allow air ambulance helicopters to land and treat the injured.

Surrey police declared a major incident at the display after the incident, which happened at about 7.30pm.

Police reported that eight children had been taken to hospital with “potentially serious injuries”. The South East Coast ambulance service said the children were transported to major hospital trauma centres in London for treatment.

But in a statement on Sunday morning, Surrey police said: “Following last night’s evacuation of Woking Park after children fell from an inflatable slide, we are pleased to update that seven of the eight victims were discharged overnight.

“The eighth victim was kept in overnight for observations, but his injuries are not thought to be significant. The investigation will now sit with the Health and Safety Executive.”

Initial reports said a helter-skelter ride had collapsed causing the injuries but Surrey police later confirmed the incident involved a large inflatable slide.

Andy Datson, 23, said he saw up to 40 children playing on the “flimsy” slide, which he estimated was about 10 metres (30ft) tall at its highest.

“We had been walking past the slide earlier in the night and said it looked unsafe. It looked pretty flimsy to say the least,” he said.

“There were far too many kids on it. It did not look as if there was an adult at the top of the slide and these kids were no more than 10 years old. It didn’t look like it could hold that many people. I would say there were about 40 kids on it.”

He said he saw several emergency vehicles arrive and “a lot of worried people” and what appeared to be children on the ground.

“It looked as if there were too many children on it and the side area had given way,” he said. “If, heaven forbid, they fell from the top, I would say it’s at least 25 to 30ft tall.”

On Twitter, Woking Rotary Club, the event’s organisers, said: “We are all very shocked and distressed by events in the funfair. We are focusing on assisting the emergency services in dealing with the injured children.”

A Twitter account run by Woking Fireworks tweeted on Saturday: “Really sorry but due to a major incident in the funfair area the police have asked us to evacuate the park.

“The fireworks display will therefore not take place tonight. We will publish further information as soon as possible.”

A Health and Safety Executive spokeswoman said initial inquiries were being made, while Woking borough council said: “We will be working with the police and relevant authorities to understand and explain what happened.”

Surrey police appealed for anyone who witnessed the incident to get in touch with them.