A suspected pipe bomb hit a Dutch coronavirus testing centre on Wednesday, shattering windows but causing no injuries in what officials branded a "cowardly act of destruction".
The blast early in the morning in Bovenkarspel, 60 kilometres (40 miles) north of the capital Amsterdam, apparently targeted health workers, Prime Minister Mark Rutte said.
The incident comes weeks after another testing centre was burned down during violent riots across the Netherlands against a coronavirus curfew.
"Police were called at 6.55 am by a security guard from the corona test centre to say that an explosion had taken place. He had heard a loud bang and then saw that several windows of the building had broken," a police statement said.
"Outside the building was a metal cylinder that had exploded. No one was injured in the incident."
North Holland police spokesman Menno Hartenberg said the explosion appeared to be deliberate but that the motive was still being investigated.
"It's not possible that this was by accident, the object was placed there and exploded near the front of the test centre," Hartenberg told AFP by telephone.
Hartenberg added: "It was a metal object somewhere between a tube and a canister."
Police forensics officers in white overalls were conducting a fingertip search of the area, which was cordoned off, an AFP journalist at the scene said.
- 'Intimidating vandalism' -
The explosion caused shock in the Netherlands, which goes to the polls in two weeks in a general election seen as a test of the government's coronavirus policies.
Prime Minister Mark Rutte said workers at the centre who "work terribly hard to keep us safe" appeared to be the targets, adding that it was "really terrible, really scandalous", according to the ANP news agency.
"For over a year now, we have relied heavily on the people on the front lines. And then this. Insane," added health minister Hugo de Jonge on Twitter.
The Dutch GGD public health department said it was "horrified" by the "aggressive and intimidating vandalism."
"Our people must be able to do this crucial work safely. This cowardly act of destruction affects us all," GGD national president Andre Rouvoet said on Twitter.
A bomb squad was sent to determine whether any explosive material remained at the scene, public television network NOS reported.
The part of the Netherlands in which Bovenkarspel is located is currently suffering one of the country's most serious outbreaks, with 81 infections per 100,000 inhabitants, compared to a national average of 27.2, NOS said.
The Netherlands has recorded more than one million coronavirus cases since the start of the pandemic and over 15,000 deaths.
Some 1.3 million people have received at least one dose of vaccine.
- Curfew riots -
In January a Covid testing centre was set on fire in the conservative protestant fishing village of Urk as protests broke out over the start of an overnight curfew in the Netherlands.
The Netherlands suffered several nights of rioting, the most violent the country has seen in decades.
An attempted arson attack on a test centre in Hilversum was also reported later in January.
The blast in Bovenkarspel came as the Dutch government on Wednesday began easing its toughest anti-virus measures since the pandemic began, ahead of elections in two weeks.
Hairdressers and other contact professions were allowed to reopen after several months of closure.
Anger over the Covid measures has been growing in the Netherlands, as well as support for conspiracy theories about the virus.
A Dutch court last month backed a case by a Covid-sceptic group saying that the curfew was illegal and must be lifted, although an appeals court later dismissed the ruling.