Police and other emergency services were called to the O2 Academy in Brixton at 21:35 pm (2135 GMT) following reports “that a large number of people were attempting to force entry to the venue.”
They arrived to find a number of people with injuries believed to have been caused by crushing.
“Detectives are investigating the circumstances which led to four people sustaining critical injuries,” the Metropolitan police said.
Two “less seriously injured” people were treated at the scene while the eight others were taken to hospital by ambulance.
No arrests have been made, the Met noted.
“This is an extremely upsetting incident which has left four people critically ill in hospital,” Commander Ade Adelekan said.
“A police investigation has been launched, and it will be as thorough and as forensic as necessary to establish exactly what happened last night.”
He added specialist officers will examine the scene and CCTV, but appealed for anyone with information to come forward.
