In January 2014, an inquest jury found that the killing of Mr. Duggan had been justified, finding it more likely than not that he had tossed a firearm from a taxi shortly before he was shot. But in October, a judge granted Mr. Duggan’s family the right to appeal those findings.

The 2011 riots echoed disturbances that shook the Brixton neighborhood of London no fewer than three times — in 1981, 1985 and 1995 — in response to anger at police treatment of black residents.

The activists have been upset by three recent deaths of black Britons: Mzee Mohammed, 18, who died last month after he was arrested in Liverpool; Sarah Reed, a 32-year-old with a history of mental illness, who died in a jail in North London in January and had previously been the victim of police brutality; and Jermaine Baker, 28, who was fatally shot by the police in North London in December, after he tried to free a convict from a police van.

“In the U.K., we have exactly the same problems as in America, but in America they’re far worse,” Kehinde Andrews, an associate professor of sociology and head of the black studies program at Birmingham City University, said in a phone interview. “You’re three times more likely to be killed by the police if you’re black, but the police don’t kill that many people in Britain in general.”

So far this year, the police in Britain have shot and killed two people, compared with 571 in the United States.

Britain has the largest prison population in Western Europe, and blacks are overrepresented. But even so, the incarceration rate is far lower than in the United States. Gun violence is also relatively rare in Britain; handguns were effectively prohibited after a 1996 massacre at a school in Scotland. Most police officers in the country do not carry guns.

Because deadly encounters with the police are fairly rare, activists in Britain have tended to focus more on the treatment of black people in custody, like Sean Rigg, a 40-year-old musician who had schizophrenia and who died in a police station in Brixton in 2008; and Kingsley Burrell, a 29-year-old student who died in police custody in Birmingham in 2011.