So far 18 people who blocked a motorway to stop an anti-Zwarte Piet demonstration taking place in the Frisian village of Dokkum in November have been summoned to attend police interviews, the public prosecution department said in a statement.

In total, 31 people are considered suspects in the investigation but only 18 have so far been identified. All come from Friesland, the department said.

Several dozen people, many thought to be members of extreme right nationalist organisations, stopped the buses carrying some 120 demonstrators by braking in front of them on the highway, forcing all traffic to a halt.

The action led to the demonstration being cancelled by the mayor on public order grounds even though the demonstrators had been given a licence to protest during the arrival of Sinterklaas.

No-one was arrested during the stand-off but police did photograph the number plates of all vehicles involved in forming the blockade.

A crowdfunding campaign set up to raise money to pay any fines has already brought in €30,000.