Three police officers have been injured during riots in Plymouth and a police van has been damaged.
Devon and Cornwall Police said six people were arrested on Monday night and work was ongoing to “de-escalate the situation”.
Several officers also suffered injuries in the violence, Devon and Cornwall Police said. Two members of the public were taken to hospital.
The force deployed 150 officers in the city centre where two protest groups formed.
Officers detained a man on the floor, as protesters launched missiles and fireworks at a counter-demonstration where people held signs stating “No Place for Hate” and “Say No to Nazis”.
Speaking at the scene, bronze commander Inspector Ryan North Moore told Sky News: “This is not a protest anymore. In my opinion, this is violence. This is sustained violence.
“Unfortunately, large masonry was thrown at us, whether it was aimed at us or others or opposing groups – we don’t know yet. The investigation will identify that. But, yes, three officers down.”
Asked how difficult it was to police the protest, Inspector Moore said: “It’s off the scale today. With the resources we’ve got, it’s difficult.”