An explosion blew out the windows of a COVID-19 testing centre in the town of Bovenkarpsel in North Holland, Dutch police said on Wednesday.
Nobody was injured in the blast, which occurred just before 7am.
Menno Hartenber, a spokesman for North Holland police, told Euronews "a small metal object or pipe, 10cm in length, was attached to the side of the COVID-19 test centre".
He told Euronews the loud blast was heard by people nearby, and it left five windows shattered.
Dutch police added that they had cordoned off the area and were "investigating the explosive and its origin".
A bomb squad was sent to see whether any explosive material remained at the scene.
"For the moment, we are not ruling out any leads and cannot yet say anything about the motives, an investigation is under way," Hartenberg said.
But he confirmed, "someone put it there", so the blast was not accidental.
Health minister Hugo de Jonge hit out at the perpetrator on Twitter, saying: "For over a year now, we have relied heavily on the people on the front lines. And then this. Insane."
"Let's stay behind the people who are working hard to get us out of this crisis," he added.
The head of the country’s umbrella organisation for local health services that carry out coronavirus testing called the blast a “cowardly act.”
“Our people have to be able to do this crucial work safely,” Andre Rouvoet tweeted.
In January, a coronavirus testing centre in the nearby town of Urk was set on fire by rioters opposed to the country's lockdown.
The lockdown measures sparked protests and civil unrest throughout the Netherlands, including clashes between riot police and protesters in Amsterdam in several other cities, where looting occurred.
Dozens of people were arrested.
The curfew, in force from 9 pm to 4.30 am until at least 15 March, is the first in the Netherlands since the Second World War.
It was ordered to be stopped last month by a court in the Hague, which said the government had abused its emergency powers by implementing it.
But the government appealed and subsequently rushed legislation through parliament to give the curfew a stronger legal backing.
The country has officially recorded more than 1.1 million coronavirus cases and 15,775 deaths, in a country with a population of around 17 million.