Three people are in critical condition after a crowd crush at a London concert.
Nigerian Afrobeat star Asake was forced to cancel his sold-out show at the O2 Academy Brixton halfway through after crowds of people, many of whom did not have tickets, attempted to gain access.
Eight people were taken to hospital. Two other injured people were treated at the scene.
Three people remained in a critical condition in hospital on Friday afternoon, down from four earlier, said police.
In a statement outside Brixton police station, Chief Superintendent Colin Wingrove said an "urgent investigation" was under way.
He said one person had been arrested on suspicion of assaulting a police officer.
Video from outside the venue shows scores of people pushing to get in.
Sky News reports fans inside were told: "The reason we have to stop the show is because they have breached the doors.
"You've got 3,000 people who have broken the doors outside and because of security the police have asked us to close the show."
Social media users said concertgoers were locked outside in freezing temperatures.
It remains unclear how long venue security had tried to contain the disruption before emergency services arrived.
The Metropolitan Police, the London Ambulance Service and the London Fire Brigade were called to the venue at around 9.30pm on Thursday.
Officers, ambulance crews and firefighters arrived and found “a number of people with injuries believed to have been caused by crushing", the Met said.
The singer posted a statement on Instagram to say he was reaching out to those affected.
He wrote: “My heart is with those who were injured last night and caused any form of discomfort. I pray you get well soonest.
“I am also in the process of reaching out to individuals. I still do not have the full brief from the venue management themselves as to what led to the disruption at the entrance of The Brixton Academy, but we are thankful that all was peaceful at the end.
“For my people who enjoyed the beginning of my performance, I am sorry that it was cut short. Thank you for your unbelievable love London. I love you too much! I’m looking forward to seeing you again in a bit!”
Asake was recently nominated for BBC Radio 1's Sound of 2023 award, for artists breaking through in the UK.
Ahead of the gig, he had posted on Twitter asking fans not to come to the venue unless they had a valid ticket.
The concert was the third of three sold-out dates at the 4,921-standing capacity venue and concluded a run of UK dates.
Footage posted online showed large crowds outside the building on Thursday night, with many people holding up mobile phones and filming the scene.
In other video content, the crowd inside the venue could be heard booing when an announcement was made from the stage saying the show was being stopped.
A woman involved in the crush, who spoke to the BBC after being admitted to hospital with suspected internal bleeding, said: "I couldn't breathe and I completely passed out. I thought I was dead.
"I took my last breath and I could not get any more oxygen. I was so scared.
"I was just queuing up to give them my ticket and then the crowd out here was insane, they just came and pushed and I just fell down."
John, 32, who lives next to the O2 and declined to give his surname, said that the police arrived after the gig was cancelled.
“It looked like the gig had been cancelled, and then some commotion, some pushing and shoving, and the police came shortly after," he said. "That’s when it kicked off and became aggressive.”
He said: “You could tell there was tension."
“People at the front of the Brixton Academy were really pushing and shoving and trying to break through the door and force their way in,” he added. “The glass of the doors was broken.”
Mr Wingrove said an incident in which an officer was "apparently seen to push a member of the public" is currently under review by the force's director of professional standards.
"This is an extremely upsetting incident which has left four people critically ill in hospital. My thoughts and prayers are with them and their families," said Met Police gold commander Ade Adelekan.
"A police investigation has been launched, and it will be as thorough and as forensic as necessary to establish exactly what happened last night.
"The scene will be examined by specialist officers, CCTV will be viewed, every witness that we can make contact with will be spoken to and all other lines of enquiry will be followed. Officers are remaining in the area to provide reassurance to the local community in and around Brixton.
“I would ask people to be sensible about what they share, and not to post material that will be upsetting to those affected by this incident,” he said.
"Where force has been used by police officers, those officers know they have to be accountable for their actions. The Met's Directorate of Professional Standards will view all material, including body-worn video footage from the officers at the scene.
"In relation to a clip being widely shared, I can confirm that no police dogs were deployed to the location."
No arrests have been made in relation to the incident, and police are urging anyone with mobile phone footage that could help with investigations to contact them.
Several members of the audience have described the incident on social media.
"I was squashed, people stampeded in after hours of standing outside and having the doors closed. I could not breathe," one Twitter user wrote.
Asake is yet to comment on the incident. The singer, 27, released his debut album, Mr Money With The Vibe, this year, and has performed several sold-out shows around the world.