"During the urgent investigative [FSB] operation, the local man was detained at his place of residence in Astrakhan," the spokesman said.
According to the FSB, a young man in his 30s swore allegiance to Daesh and planned "an armed attack on administrative buildings."
The FSB also seized weapons, ammunition, grenades and extremist literature from the detainee.
A criminal case was opened under articles related to terrorism and participation in a terrorist organization.
*Daesh (also known as IS/ISIS/ISIL/Islamic State) is a terrorist group banned in Russia.