A 20-year-old man has died after being apprehended by a police officer in an east London shop.
The Met Police said the man, named by his family as Rashan Jermaine Charles, was followed on foot after officers tried to stop a car in Kingsland Road, Hackney, at 01:45 BST on Saturday.
Mr Charles was taken ill after trying to swallow an object and was pronounced dead in hospital, police said.
The Independent Police Complaints Commission (IPCC) is investigating.
Footage apparently showing the incident has been shared on social media, along with the hashtag #JusticeForRash.
The film, recorded by a security camera, shows Mr Charles entering a shop pursued by a uniformed police officer.
In the footage, there is a struggle on the floor, and Mr Charles appears to put his hand to his mouth.
Another man in plain clothes is seen helping the officer. Mr Charles is seen handcuffed with his hands behind his back.
Scotland Yard said the officer "intervened and sought to prevent the man from harming himself".
A force medic provided first aid at the scene before London Ambulance Service paramedics arrived.
Mr Charles was taken to the Royal London Hospital in east London and pronounced dead at 02:55 BST.
Police said next of kin had been informed and a post-mortem examination would be held.
A makeshift memorial to Mr Charles has sprung up by the scene of the incident as members of the local community have laid flowers and lit candles outside the shop.
Simon Laurence, the Met's borough commander for Hackney, said: "There is likely to be speculation over the next few days regarding what led to this man becoming ill, so I would encourage people to keep up-to-date with the IPCC's statements, as and when they are released.
"All police officers are fully aware that they will be asked to account for their actions - officers are not exempt from the law and we would not wish to be."
The IPCC confirmed it had begun an independent investigation, taking evidence from eyewitnesses and police officers.
It said CCTV footage from inside the shop and police body-worn video evidence had been gathered and viewed.
"The IPCC has obtained evidence which indicates an object was removed from [Mr Charles's] throat at the scene," a spokesman said.
He appealed for information from witnesses who were in the Kingsland Road and Middleton Road area of Hackney.
Campaigners from Hackney Stand Up To Racism have announced a vigil for Mr Charles outside Stoke Newington police station on Monday evening.