Hundreds of people have gathered at Windrush Square in Brixton, south London, for Afrikan Emancipation Day.
A number of demonstrators temporarily blocked Brixton Road at the junction with Acre Lane, forcing cars and buses to stop and turn around.
Protesters, including some from Extinction Rebellion, occupied the middle of the junction until they were told to get back on to the curb by police officers.
A short distance away, another group observed speeches before a three-minute silence was held to mark Afrikan Emancipation Day.
A coalition of action groups is due to attend the event, including Stop The Maangamizi: We Charge Genocide/Ecocide, the Afrikan Emancipation Day reparations march committee and the Rhodes Must Fall Oxford campaigners.
Extinction Rebellion said the event aims to promote the necessity of stopping the genocide and ecocide of African people and their environments.
Three people - holding signs saying “mask up” and wearing visors - handed out face masks and hand sanitiser to those attending. Photos, however, appeared to show many people not adhering to social distancing rules.
Numerous groups were there primarily to enjoy a family-friendly, socially distanced day of activities and learning, police said, but warned the event must not spill into nearby roads and must finish by 8pm.
The force said that the time limit was set so that officers could separate those attending the demonstrations from people attending other gatherings or unlicensed music events.
Deputy Assistant Commissioner Laurence Taylor, in charge of the policing operation on Saturday, said: “The decision to impose conditions on an assembly is never taken lightly and is made following a vigorous assessment of the information available to us.
“We have no intention of infringing upon a responsibly-organised community event.
“We only require that this is done in a way that makes use of Brixton's open spaces and leaves the main road through Brixton open to other Londoners.
“In recent weeks we have policed a number of UMEs (unlicensed music events) in which loud music is played at night, disrupting local residents and posing a real threat both to property and the officers who attend to disperse them.
“We have received information that there are those intending to come to Brixton on Saturday to purposely cause disruption, and to confront police officers.
“This is in stark contrast to the feel of the events that will take place earlier in the day and is in opposition to the wishes of the local community.”
Scotland Yard said that gatherings of more than 30 people will be in breach of the Health Protection (Coronavirus) Restrictions and its decision to impose conditions did not mean the assembly in breach of these regulations was authorised by police.