BARCELONA, Spain — The conflict with the restive northeastern region of Catalonia has entered a “new phase” with the sentencing of its former separatist leaders to prison this week, said Spain’s caretaker prime minister, Pedro Sánchez.

By Friday, it was clear that phase had indeed begun, but not in the way that Mr. Sánchez had in mind.

The violence on the streets of Barcelona and other Catalan cities since the Spanish Supreme Court ruling on Monday has raised fears that Catalonia’s separatist politicians are now losing control over their supporters, some of them growing increasingly radicalized.

On Friday, the Catalan independence movement delivered a new show of force in Barcelona. Five columns of separatist protesters marched from different parts of the region into the city amid a general strike. Friday afternoon’s demonstration drew about 525,000 people, according to local police.