A woman in her 40s is in hospital with life-threatening injuries after a funfair ride malfunctioned at a country show in south London, the Metropolitan Police said.
A man in his 40s is also receiving treatment for potentially life-changing injuries.
The Lambeth Country Show in Brockwell Park reopened on Sunday, but rides remain cordoned off.
An 11-year-old girl, the man and woman aged in their 40s, and a man aged in his 50s, were taken to hospital after the incident.
The man in his 50s and the 11-year-old girl have both been discharged from hospital, Scotland Yard said.
The problem occurred during the first day of the free event.
A ride topped with a giant frog is among those which could be seen behind tall black fencing on Sunday.
A police spokesperson said: “Emergency services responded at around 6.20pm on Saturday June 8 to a fairground ride having failed at the Lambeth Country Show in Brockwell Park.
“Four people have been taken to hospital. Following a full assessment by medics, a woman in her 40s remains in hospital with life-threatening injuries. A man in his 40s is also receiving ongoing treatment for potentially life-changing injuries.
“The man in his 50s and the 11-year-old girl have both been discharged from hospital. Officers have attended the scene today alongside the Health and Safety Executive as inquiries continue to establish the full circumstances.”
Signs saying “apologies for any inconvenience, this ride will not open today” have been pinned to wire fencing which circles the fairground area.
Disappointed visitors of all ages could be seen reading the sign and talking to a fairground worker, who told them all the rides are shut, before heading off to enjoy other attractions at the event.
An investigation into the incident, led by the Met Police and Health and Safety Executive, is under way, Lambeth Council said.
A spokesperson added: “Very sadly a funfair ride at the country show malfunctioned yesterday evening injuring four people.
“They are being cared for in hospital and our thoughts are with them.
“We thank the emergency services for their swift response at the country show yesterday and for the support they are providing to the affected families.
“Following this incident, and to allow a full investigation, funfair rides at the show will be impacted by closures for the second day of the country show.”
The council-organised show has been running as a free community event in Brockwell Park, south London, since 1974.
Annually it attracts more than 120,000 people across the weekend.
Helen Hayes, the Labour parliamentary candidate for Dulwich and West Norwood, posted on X, formerly Twitter, on Saturday evening: “I’m shocked to learn of the failure of a fairground ride at Lambeth Country Show earlier this evening.
“My thoughts are with those who were injured and all who witnessed this horrific event.
“Thank you to the emergency services who attended.
“There must now be an urgent investigation.”
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