Three people were being treated in hospital in Rotterdam on Saturday after they were seriously injured when police fired shots during a demonstration against Covid-19 restrictions.
In what the Dutch city's mayor described as an "orgy of violence", 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 cannon.
Police said on Twitter on Saturday that 51 people had been arrested, about half of whom were under 18.
"Three rioters were wounded when they were hit by bullets, they remain in hospital," police added, in an update after earlier reporting two wounded.
The 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 rampaged through the shopping district, starting fires and throwing stones and fireworks at officers.
"They shot at protesters. People were injured," Aboutaleb said.
Photos in Dutch media showed at least one police car on fire and another with a bicycle smashed through the windscreen. Police said city authorities had closed the city's main railway station.
Riot police and the use of water cannon quelled the unrest after midnight.
There were further violent scenes around the Netherlands on Saturday, when largely peaceful daytime protests turned ugly as night fell. Rioters pelted police with stones and fireworks in The Hague, while officers in riot gear charged groups of demonstrators. Five police officers were injured and at least seven people arrested.
Riots also erupted on Saturday in the central "Bible Belt" town of Urk and cities in southern Limburg province, while angry fans disrupted two football matches being played behind closed doors because of coronavirus rules, Dutch media said.
It was one of the worst outbreaks of violence in the Netherlands since coronavirus restrictions were first imposed last year. In January, rioters attacked police and started fires on the streets of Rotterdam after a curfew came into force.
The Dutch justice minister, Ferd Grapperhaus, condemned the events. "The riots and extreme violence against police officers, riot police and firefighters last night in Rotterdam are disgusting to see," he said in a statement.
"Protesting is a great right in our society, but what we saw last night is simply criminal behaviour. It has nothing to do with demonstrating."
An independent investigation into the shootings by police has been opened, as is the case whenever officers use their weapons.
The local political party Leefbaar Rotterdam condemned the violence."The centre of our beautiful city has this evening transformed into a war zone," it said on Twitter. "Rotterdam is a city where you can disagree with things that happen, but violence is never, never the solution."
The government has said it wants to introduce a law that would allow businesses to restrict the country's coronavirus pass system to people who are fully vaccinated or have recovered from Covid-19, which would exclude people who test negative.
The country has reported record numbers of infections in recent days and a new partial lockdown came into force a week ago. Cases are rising across Europe because of a combination of low vaccine uptake, waning immunity among people already inoculated and growing complacency about masks and distancing after governments relaxed curbs over the summer.
According to figures from OurWorldInData, the EU's average has quadrupled in recent weeks, from just over 110 new daily cases per million people on 1 October to 446 on Thursday.
The government has also banned fireworks on 31 December for the second consecutive year. The ban is intended "to prevent, as much as possible, extra strain on healthcare, law enforcement and first responders", it said on Friday.
Organisers of a protest planned in Amsterdam against the coronavirus measures on Saturday said they had cancelled the event after Friday's violence.
