Four people were critically injured and four others were hospitalised following a crush outside a south London venue hosting a concert by Nigerian Afrobeats singer Asake, police said Friday.
Emergency teams were called to the O2 Academy in Brixton at 21:35 pm (2135 GMT) on Thursday following reports "that a large number of people were attempting to force entry to the venue".
Police, ambulance crews and fire brigade officers arrived to find numerous people with injuries.
Eight people were taken to hospital by ambulance while two "less seriously injured" were treated at the scene.
The incident occurred during the last of three sold-out performances at the venue by singer-songwriter Asake.
The gig was cancelled midway through the performance. Reports said an irate crowd was told that up to 3,000 people had tried to break through the doors, with some succeeding, and that police had ordered the show to stop.
"Detectives are investigating the circumstances which led to four people sustaining critical injuries in Brixton," the Metropolitan police said in a statement, noting no arrests have been made.
Commander Ade Adelekan called the incident "extremely upsetting" and vowed an investigation "as thorough and as forensic as necessary to establish exactly what happened last night".
He added that specialist officers will examine the scene and CCTV, but appealed for anyone with information to come forward.
Video clips posted on social media and featured on tabloid newspaper websites appeared to show packed crowds pushing up against the doors of the 02 Academy, as well as violent scuffles in the foyer.
It was unclear who was involved in the clashes, but Adelekan noted "where force has been used by police officers, those officers know they have to be accountable for their actions".
He said the Met's Directorate of Professional Standards, a policing watchdog, will view all material, including body-worn video footage from officers at the scene.
The O2 Academy could not be immediately reached for comment.