In this image taken from video, demonstrators protest against government restrictions in Rotterdam. AP

Netherlands police opened fire on protesters in rioting that erupted in downtown Rotterdam around a demonstration against COVID-19 restrictions. The Dutch city's mayor called it "an orgy of violence,” in which seven people were wounded and more than 20 arrested.

Mayor Ahmed Aboutaleb told reporters in the early hours of Saturday morning that "on a number of occasions the police felt it necessary to draw their weapons to defend themselves” as rioters ran rampage through the port city's central shopping district, setting fires and throwing rocks and fireworks at officers.

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Crowds of several hundred rioters torched cars, set off fireworks and threw rocks at police during the protests on Friday evening. Police responded with warning shots and water canons.

This photograph shows burned bikes after a protest against the partial lockdown in Rotterdam, Netherlands. AFP

"Police were forced to draw their weapons and even fire direct shots," Mayor Ahmed Aboutaleb told a press conference early on Saturday.

"They shot at protesters, people were injured,” Aboutaleb said. He did not have details on the injuries. Police also fired warning shots.

A number of police officers also were injured in the violence and officers arrested dozens of people and expect to arrest more after studying video footage from security cameras, Aboutaleb said.

In this image taken from video, demonstrators protest against government restrictions in Rotterdam. AP

Photos from the scene showed at least one police car in flames and another with a bicycle slammed through its windshield.

Riot police and a water cannon restored calm after midnight.

It was one of the worst outbreaks of violence in the Netherlands since coronavirus restrictions were first imposed last year. In January, rioters also attacked police and set fires on the streets of Rotterdam after a curfew came into force.