Danish police have detained several people and searched several premises as part of an ongoing investigation into political extremism.

Those detained are believed to be affiliated to the left-wing Antifascist Action (Antifascistisk Aktion) group.

The Security and Intelligence Service (PET) says that among other items, a computer server has been seized containing sensitive personal information about the group’s opponents, right-wing parties and politicians.

According to the reports, the information is thought to have been collated using data crime.

PET says that over the past several years it has registered an increasing number of clashes between extremists from both sides of the political spectrum.

“The confrontations seem to have an increasing character of organised violence and we have seen extremists from the right and left trying to uncover and expose political adversaries using activities that resemble private intelligence operations,” says PET Chief Jakob Scharf.

One of those detained has been remanded in custody for 24 days charged with violence and political violence for having supported a group that is trying to exert pressure on society using force.

It is not immediately clear how many people were detained.