THE HAGUE: The mayor of Rotterdam on Saturday condemned "an orgy of violence" at protests against COVID-19 measures in the Dutch port city, in which seven people were wounded and more than 20 arrested.

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.

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

It was "an orgy of violence," he said. "I can't think of another way to describe it."

Authorities said they had arrested more than 20 people and expected to detain others, as the city centre where the riots took place is extensively monitored by security cameras.

Dutch justice minister Ferd Grapperhaus said in a statement that the "extreme violence" against police and fire fighters in Rotterdam was "repulsive".

"The right to protest is very important in our society but what we saw last night was simply criminal behaviour," Grapperhaus said.

Protesters had gathered to voice opposition to government plans to restrict access to indoor venues to people who have a "corona pass", showing they have been vaccinated or have already recovered from an infection.

The pass is also available to people who have not been vaccinated, but have proof of a negative test.

Organisers of a planned protest Saturday in Amsterdam against the coronavirus measures said they had canceled the event after Friday's violence.

A musical protest called by DJs in the southern city of Breda against current COVID-19 measures, which include the 8 p.m. closure of bars, restaurants and clubs, will go ahead. Organisers say they expect several thousand people.

The Netherlands re-imposed some lockdown measures last weekend for an initial three weeks in an effort to slow a resurgence of coronavirus contagion, but daily infections have remained at their highest levels since the start of the pandemic.