Riot police fired teargas in the French capital to disperse a largely peaceful, but unauthorised, protest against police brutality and entrenched racism.
At least 15,000 people rallied in Paris, led by supporters of Adama Traore, a French black man who died in police custody in 2016.
In London, far-right activists clashed with police after they rallied around statues of famous Britons, whose role in history is marred by slavery or actions for the British Empire.
The European rallies are, in part, reactions to the killing in the US of George Floyd, as a police officer tried to arrest him. His death sparked protests around the US and across Europe.
Police fired teargas at anti-racism protesters in Paris shortly after a small group of counter-protesters unfurled a banner from a building denouncing "anti-white racism", and managed to keep the rival groups apart.
Hundreds of other protesters took a knee – a symbolic protest against police brutality - and stayed for hours despite the police pressure.
Myriam Boicoulin, 31, a black woman born in the French Caribbean island of Martinique who now lives in France, said she marched on Saturday because she "wants to be heard."
“The fact of being visible is enormous,” Ms Boicoulin said. “I’m constantly obliged to adapt, to make compromises, not make waves, to be almost white, in fact. It’s the first time people see us. Let us breathe.”
Similar protests were also held Saturday in cities around France, from Rouen in Normandy in the northwest to Marseille on the Mediterranean
"We are are all demanding the same thing - fair justice for everyone," Traore's sister Assa told the Paris rally.