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 Met Police has said.
The incident happened during Lambeth Country Show in Brockwell Park at about 18:20 BST on Saturday.
A man in his 40s is also being treated for "potentially life-threatening injuries", the force said.
Lambeth Council said the investigation would "determine the cause of the malfunction".
Two other people, a man in his 50s and an 11-year-old girl, were injured in the incident and have since been discharged from hospital, the Met said.
The problem occurred during the first day of the free event.
Funfair rides at the show were closed on Sunday, the council added.
One of the fairground rides was seen covered in tarpaulin on Sunday morning, although it was not clear which ride had malfunctioned.
The council said emergency services had arrived at the scene shortly after the incident and the funfair had been closed.
Officers attended the scene on Sunday alongside the Health and Safety Executive.
The force said that "inquiries continue to establish the full circumstances".
Additional safety checks were being carried out on all rides and attractions at the show, Lambeth Council said.
A spokesperson for the council added their thoughts were with the four people injured on the ride.
“We thank the emergency services for their swift response and for the support they are providing to the affected families," they said.
"The safety and well-being of our attendees remains our top priority, and we are committed to providing a secure and enjoyable experience for all visitors.”
The show continued from 13:00 BST on Sunday but with all fairground rides shut and cordoned off.
Signs that said "apologies for any inconvenience, this ride will not open today" were pinned to wire fencing circling the fairground area.
Visitors of all ages were seen reading the signs and talking to a fairground worker, who told them all the rides are shut.
Brockwell Park is a popular green space located near Brixton.
The incident fell on the 50th anniversary of the Lambeth Country Show, which is one of the "biggest free family festivals in the UK", according to its website.
A spokesperson for London Ambulance Service said four crews were deployed along with members of its hazardous area response team and an air ambulance.
All four people injured were taken to "major trauma centres", they added.
