Five far-right extremists suspected of planning a terrorist bomb and gun attack have been remanded in custody in Finland, police said on Friday.

The five men from Kankaanpaa in the southwest were aged "around 25" and were arrested on Tuesday morning after being under surveillance for two years, police told a news conference.

Detective Superintendent Toni Sjoblom, head of investigation, at the Southwestern Finland police department, shows some of the material found during the arrest of five far-right extremists Detective Superintendent Toni Sjoblom, head of investigation, at the Southwestern Finland police department, shows some of the material found during the arrest of five far-right extremists Photo: LEHTIKUVA via AFP / Juha Sinisalo

Material found in the men's possession "reinforces the impression that they have become radicalised and gives reason to suspect them of terrorist offences", Detective Superintendent Toni Sjoblom said in a statement.

"A significant quantity of guns, ammunition and explosives" were recovered during a home search in December 2019 when the men were arrested on firearms charges, Sjoblom said.

According to police the men appear to be motivated by "accelerationism", a white supremacist ideology that has been linked to mass shootings in the US, in which followers aim to sow division and inter-racial tensions in society.

Detective Superintendent Toni Sjoblom, head of investigation, at the Southwestern Finland police department, shows some of the material found during the arrest of five far-right extremists Detective Superintendent Toni Sjoblom, head of investigation, at the Southwestern Finland police department, shows some of the material found during the arrest of five far-right extremists Photo: LEHTIKUVA via AFP / Juha Sinisalo

Police declined to reveal the planned target of the attack but said the arrests marked Finland's first case of suspected far-right terrorism.

There was no danger to the public, police said.

Residents of Kankaanpaa who know some of the suspects told Finnish newspaper Helsingin Sanomat that one was a "skinhead" and that two others were "known as neo-Nazis".

Finnish Security and Intelligence Service specialist researcher Eero Pietila (L, pictured with Detective Superintendent Toni Sjoblom) said the case has been "a key driver of... threat assessment Finnish Security and Intelligence Service specialist researcher Eero Pietila (L, pictured with Detective Superintendent Toni Sjoblom) said the case has been "a key driver of... threat assessment" Photo: LEHTIKUVA via AFP / Juha Sinisalo

Police said that the EU's law enforcement agency Europol was also involved in the investigation, but that the group did not appear to belong to an extremist organisation.

"A small group like this which idealises terrorist violence works in secret and their activity does not involve contact with organised publicly operating extreme-right groups," Pietila said.

Photos released by the police showed one of the suspects in a balaclava posing with a handgun and a machete, while a second photo showed weapons confiscated from the group including a semi-automatic rifle, homemade explosive devices and bundles of dynamite.

In an update in March, Finland's security services Supo said they would keep the terrorist threat level at "elevated", the second-lowest level on a four-point scale.

However, they warned that the risk from far-right extremism was "more worrying" than in the previous year.

"This case has been a key driver of that threat assessment," Supo specialist researcher Eero Pietila said.

Terrorism arrests in Finland are rare.

The country's first prosecution on terrorist charges came in 2018, when a rejected Moroccan asylum-seeker, Abderrahman Bouanane, stabbed 10 people, killing two, in the southwest town of Turku.

Police said that the pre-trial investigation into the five suspects will take a number of months, with prosecutors giving the police until March 31 next year to press charges.