South Korean authorities have arrested three members of a small, left-leaning political party and charged them with conspiring to stage a rebellion.
The state intelligence agency on Wednesday raided homes and offices belonging to officials of the Unified Progressive Party, including one of its members of parliament.
Representative Lee Seok-ki was not charged because he has parliamentary immunity. But three others were charged under South Korea's strict National Security Law.
South Korean media, including YTN and KBS television, reported that the alleged plot including plans to attack a national communications center and other infrastructure.
Some conservatives in Seoul have accused the UPP of supporting North Korea, a charge the party denies.
UPP officials on Wednesday said the National Intelligence Service's investigation was an attempt to distract the public from allegations that the spy agency attempted to rig last year's presidential election.