An explosive went off at a coronavirus testing centre north of Amsterdam in the Netherlands early this morning, shattering windows but causing no injuries in what police called an intentional attack.

An explosives team was on site in the town of Bovenkarspel, 55km north of the capital, to examine the device, police in the province of North Holland said.

The metal remains of the explosive, about 10 cm by 10 cm in size, were found in front of the building and "must have been placed" there, a police spokesman said.

"We don't know yet exactly what exploded, the explosives experts must first investigate," he said, adding, "what we're saying is that something like that doesn't just happen by accident, it has to be laid."

A security guard who was inside the testing centre at the time was not injured, national broadcaster NOS reported.

The incident comes shortly before national elections on 17 March widely seen as a referendum on the government's handling of the pandemic.

Anger against healthcare authorities has increased since the start of 2021, with the head of the country's National Institute for Health now accompanied by a security detail.

Another test location was burned down during several days of rioting in January sparked by the introduction of a night-time curfew from 9pm to 4.30am.

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The region around Bovenkarspel, a rural town, is currently suffering one of the Netherlands' worst Covid-19 outbreaks, with 181 cases per 100,000 inhabitants, compared with around 27 per 100,000 nationally.

At least one hospital has been forced to send patients to other provinces due to lack of space in its intensive care units.

Today is the first day in several months in which lockdown measures in the Netherlands have been slightly eased, with hairdressers reopening and non-essential stores allowed to accept a small number of visitors by appointment.