Three people were being treated in hospital in Rotterdam after they were seriously injured when Dutch police fired shots during a violent protest against COVID-19 measures, authorities said.
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.
Rotterdam police posted on Twitter on Saturday that 51 people had been arrested, 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.
Authorities are investigating the shootings including whether the wounded people were hit by police bullets, they added.
The city's mayor, Ahmed Aboutaleb, said "police were forced to draw their weapons and even fire direct shots".
Dutch Justice Minister Ferd Grapperhaus said in a statement the "extreme violence" against police and firefighters 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.
In several towns in the Netherlands on Saturday evening there were small pockets of unrest as youths clashed with police.
In The Hague, riot police used water canons to clear part of the central Schilderswijk area after rioters threw fireworks at police and damaged traffic lights, local police said on Twitter.
There was a heavy police presence in several other major towns after social media calls to riot followed the Rotterdam clashes, but any further violence was largely contained, with only a handful of arrests, Dutch media reported.
Organisers of a protest that had been planned in Amsterdam on Saturday against the coronavirus measures said they had canceled the event after Friday's violence.
Even so, several thousand protesters showed up for a march around the city's central Dam square. The march remained peaceful, monitored by a heavy police presence.
The Netherlands reimposed 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.