The blast in the town of Bovenkarspel, 55km north of the capital, shattered windows but caused no injuries, police from the province of North Holland said in a statement.

They said they had cordoned off the area to investigate.

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

Prime Minister Mark Rutte's conservative VVD Party is likely to remain the largest, according to opinion polls.

The metal remains of the explosive, about 10cm by 10cm in size, were found on the front of the building and "must have been placed" there, police spokesman Menno Hartenberg told Reuters.

"We don't know yet exactly what exploded, the explosives experts must first investigate," Hartenberg said.

"What we're saying is that something like that doesn't just happen by accident, it has to be laid," the spokesman said.

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.

Wednesday marks 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. A night-time curfew from 9pm to 4:30am remains in place.

Anger against healthcare authorities has increased since the start of 2021 and the head of the country's National Institute for Health is now accompanied by a security detail to public appearances.

Australian Associated Press