Demonstrators chanted “black lives matter” and “racist police, our streets” as they drew large crowds to Windrush Square in Brixton.
Traffic was halted for at least four hours as local buses were re-routed, and there were reports of scuffles between protesters and disgruntled drivers.
The sit-in protest developed after hundreds marched from the square to the local police station, calling for justice over the fatal shootings of Philando Castile in Minnesota and Alton Sterling in Louisiana.
A police helicopter flew overhead and scores of officers were deployed to the edge of the protest, but with tensions running high police seemed reluctant to get too close.
Some protesters cheered as one man shouted through a loudspeaker “we have locked down Brixton”.
Another man, Vincent Lee, 37, and his partner Jessica Osibona told the Press Association they came to the march as a mark of solidarity of the events in the US over the past week.
Mr Lee said: “It is a mess. Predominantly it is happening in America with their gun laws but we came down to show we are united with them.”
He added: “It is hard because I’m mixed. I’ve got white family members too. The officers who got killed in Dallas, they’ve got families too.”
Ms Osibona said she felt “compelled” to join the protest having followed the Black Lives Matter movement over the past year.
She said: “I was looking for some kind of inspiration as to what we can do collectively.”
She added it was her first such protest and had given her an “overwhelming feeling of unity”.
Dozens of buses and cars were forced to queue back along Brixton Road as the protesters blocked traffic outside the Ritzy Cinema. One bus driver said he had been stuck for more than two hours.
Scuffles broke out at one point after protesters said a bus driver made a derogatory remark towards them. He was punched by one protester after being ushered away by six or seven police officers. One officer told the PA he believed the driver had been arrested.
Officers then backed away from the crowd who remained in the road playing music.
The protest was one of a number planned in the UK since the shooting incident at a Black Lives Matter rally in Dallas on Thursday night, when five white police officers were killed.
A suspect in the shooting was later killed by a police-operated explosive, and named as 25-year-old Micah Johnson.
On Friday, sections of central London came to a standstill as protesters marched in the Houses of Parliament, in a rally organised by the Black Lives Matter London movement.
And another protest has been organised for Sunday morning, beginning in Oxford Circus at 10am.