Pulsing with outrage, arms lifted high, they chorused: "hands up - don't shoot!"
A thousand young, loud and angry marchers paralysed parts of central London in protest at police racism on 26 November.
Anti-racist groups called the US embassy demonstration in response to American cop Darren Wilson escaping criminal charges. Wilson killed unarmed black teenager Michael Brown in August.
Vigil speakers included relatives of Mark Duggan, victim of a racist police murder in Tottenham. The rally ended with a seemingly unplanned surge onto the streets - destination: police headquarters.
Lively, fast-moving protesters poured onto Oxford Street. Traffic and Christmas shoppers were brought to a standstill in the prickly evening mist.
Small squads of surprised police ran up and down, desperately trying to assert some authority. Bystanders whooped and saluted with raised fists; groups of young people tore leaflets from our hands. Black and Asian drivers rolled down their windows to join in.
Marchers shut Charing Cross police station and aired their displeasure at Downing Street, before heading to fenced-off Parliament Square. Overwhelmed cops chased, tripped and shoved protesters - including an elderly woman - for trying to enter. But they could not keep the cheering, charging crowd out.
The march stopped at New Scotland Yard, home of the Metropolitan Police, chanting "who killed Mike Brown? Police killed Mike Brown!"
Youthful demonstrators - many fresh from student protests the week before - were enthusiastic and open to socialist ideas. One young black woman giving the Socialist Party her contact details summed it up:
"Capitalism, racism: it's all about making money off the backs of others."
Vibrant and determined protests are important to the fight against police racism and violence on both sides of the Atlantic. We say they should be organised with stewards to defend against police attack, and linked to an ongoing strategy. We need democratic community control of policing - and a new party for workers, youth and all oppressed groups.