A group of so-called sovereign citizens, some of whom are appearing in court on Wednesday, were planning to firebomb the house of Leeuwarden mayor Sybrand van Haersma Buma, current affairs programme Nieuwsuur reported.

In a tapped phone conversation which took place in May, three of the suspects discussed placing a bomb underneath a car parked in front of Van Haersma Buma’s home and “a little bullet through Buma’s window”, according to court documents seen by Nieuwsuur.

They also referred to IRA bombing tactics and ways of disrupting the Nato summit in The Hague. Police arrested the three suspects two weeks ahead of the event.

Another five suspects, aged between 39 and 74, were arrested on the same day. Castor beans, the source ingredient for deadly poison ricin, were found at the home of one of the suspects along with weapons, military outfits and gas masks.

Sovereign citizens believe they are not held by any law and refuse to pay taxes, fines or healthcare premiums. According to security service AIVD, the police and anti-terrorist unit NCTV, their number has grown to some tens of thousands in the last few years.

A small number of them are prepared to use violence to undermine democratic rule of law, the AIVD said.

The eight men who were arrested are accused of belonging to the radical wing of the movement. Five of them will appear in court on Wednesday.

Lawyer Stephanie Heukers, who represents one of the suspects present at the phone conversation, said her client “believes he is not part of any group whatsoever. I had to explain to him what a sovereign citizen is,” she told Nieuwsuur.

She said he “abhors violence” and was not planning an attack. “If someone talks about violence it does not mean my client agrees with it,” she said.

Sybren van Haersma Buma declined to comment.