Campaigners chanted "no justice, no peace" outside a police station in protest over the death of a black man after a police chase.
Up to 150 people, many holding Black Lives Matter banners, brought traffic to a standstill outside east London's Stoke Newington Police Station in a vigil for Rashan Charles.
Unverified footage on social media appeared to show at least one police officer attempting to restrain Mr Charles on the floor of a shop, in Kingsland Road, east London, on Saturday at 1.45am.
The 20-year-old died later in hospital.
The vigil was organised by Stand Up To Racism and campaigners say they are "enormously concerned and angered" over his death.
The crowd, which was made up of people from different races and ages, listened to speeches about alleged police brutality as uniformed officers looked on.
Metropolitan Police said Mr Charles was seen "trying to swallow an object" and that an officer "sought to prevent the man from harming himself".
The police watchdog, the Independent Police Complaints Commission (IPCC), is investigating.
In a statement, Chief Superintendent Simon Laurence, the Borough commander for Hackney, said: "All police officers understand that they will be asked to account for their actions and they would not want it any other way.
"I understand his death has had an impact on some members of the local community."
He also said he had earlier met with community representatives and wanted to continue to hear about community concerns.
The IPCC tweeted: "For those following #justiceforrash #justiceforrashman - We are making good progress, building a full picture of what happened and why.
"We know people have concerns, but our independent investigation will be thorough, rigorous & when appropriate its findings will be published."
The crowd marched peacefully from the police station, behind a line of uniformed officers, to the shop where the incident happened.
Dalston resident Joyce Folks, 67, joined the walk to the shop as it passed by. She said: "I am here as a mother, grandmother and great-grandmother and it is as all those things that when I saw that video, I cried for that boy.
"My condolences are with the family. We are out here in numbers because it is the only way we can show that we care. It is a sign of community spirit. This needs to be peaceful."
The crowd walked back to the station, but a few people that stayed behind threw bottles and sticks at police at around 8.15pm.