Dutch rioters who attacked police and destroyed property over the weekend while protesting new coronavirus measures are “criminals,” Prime Minister Mark Rutte said Monday, as law enforcement officials warned that the violence could last for weeks.

The unrest across the Netherlands, some of the worst in decades, had “nothing to do with protest,” Rutte, who resigned last week following a scandal, told reporters outside his office in The Hague, news agencies reported.

“This is criminal violence and we will treat it as such,” he said.

Protesters had gathered in defiance of lockdown orders in at least 10 towns and cities Sunday, looting stores and clashing with police after authorities imposed a new nighttime curfew — the first in the Netherlands since World War II.

The violence continued Monday night in several cities, including Amsterdam and The Hague.

The curfew, from 9 p.m. to 4:30 a.m., tightens an already-strict lockdown aimed at curbing coronavirus infections and comes amid fears that a new, more contagious variant, first identified in Britain, will cause a surge in cases.

Bars and restaurants have been closed in the Netherlands since October, and schools and shops were shuttered in December. The government has recorded some 944,000 coronavirus infections and more than 13,500 deaths.

On Sunday, police deployed dogs, a water cannon and mounted officers to disperse a demonstration in central Amsterdam, arresting nearly 200 people, Reuters reported. In the fishing village of Urk Saturday, youths torched a coronavirus testing center and clashed with local media and police.

Some of the worst violence flared in Eindhoven in the south, where rioters threw stones, knives and fireworks at police and damaged the local railway station, Dutch media reported.

A far-right anti-immigration group, Pegida, had previously called for a demonstration in the city and said it would use the protest to burn copies of the Koran. The protest in Amsterdam was also organized by anti-lockdown groups on the messaging app Telegram, according to local media.

“My city is crying, and so am I,” Eindhoven Mayor John Jorritsma told media Sunday night, the Associated Press reported. He called the rioters “the scum of the earth,” adding that he was afraid the country was headed toward “civil war.”

On Monday, a national police union spokesman said officers were preparing for more unrest.

“I hope it was a one-off,” Koen Simmers said of the unrest this weekend, in an interview with the Dutch program Nieuwsuur. “But I’m afraid it was a harbinger for the coming days and weeks.”

“The police are well-prepared,” he said. “But I hope that it is not necessary.”