"The activity of a supporter of the Right Sector extremist organisation, who planned a terrorist attack near the building of Murmansk's local authorities using an improvised explosive device, has been thwarted," the FSB said in a press release.
The detained person is a Russian national.
He was detained on Tuesday in Murmansk when he was "taking a powerful improvised explosive device out of a cache."
A criminal case was opened.
The Right Sector is a Ukrainian association of nationalist organizations that took an active part in the anti-government protests that led to the ousting of former President Viktor Yanukovych in February 2014.