Two policemen were injured after a rally in the city of Bristol in southwest England against a government bill that would increase police powers to deal with protests turned violent on Sunday.
During the "Kill the Bill" rally, a police station was also attacked and at least two police vehicles were set on fire.
UK Home Secretary Priti Patel described the events as "unacceptable".
"Thuggery and disorder by a minority will never be tolerated. Our police officers put themselves in harm's way to protect us all," the minister said.
In a statement, the local Avon and Somerset Police said the peaceful protest was turned into a violent disorder by a small minority of demonstrators.
"What started out as a peaceful protest has been turned by a small minority into a violent disorder. These scenes are absolutely disgraceful and they will be widely condemned by people across the city. There can never be any excuse for wanton disorder," Chief Superintendent Will White said.
"Officers have been subjected to considerable levels of abuse and violence. One suffered a broken arm and another suffered broken ribs. Both have been taken to the hospital. These are men and women out there with the intention of serving and protecting the public - they should never be subjected to assaults or abuse in this way," he said.
The force said that at least two police vehicles have been set on fire and damage has been caused to the outside of the police station. Uniformed officers, carrying riot shields, were seen tackling the blaze.
"We have requested mutual aid from neighbouring forces to bring this incident to a safe conclusion. All those involved in this criminal behaviour will be identified and brought to justice. There will be significant consequences for behaviour such as this," White said.
Several hundred people are said to have congregated outside Bridewell police station in Bridewell Street, Bristol, since Sunday afternoon. The police said missiles were thrown at them, including a firework, and that they were verbally abused.
Bristol Mayor Marvin Rees said it was "a shameful day in an incredible year" for the city.
Ignoring the ban on mass gatherings under the coronavirus legislation, campaigners have been staging a number of protests against the new bill tabled in parliament that would give the police more powers to deal with non-violent demonstrations.
Anyone violating the coronavirus regulations could be fined. However, there have been growing calls for the law to be changed to allow peaceful demonstrations.