SEOUL, South Korea (CNN) -- A South Korean lawmaker has been sentenced to 12 years in prison for plotting an armed rebellion.
The verdict makes Lee Seok-ki -- a member of South Korea's left-wing United Progressive Party -- the country's first sitting lawmaker to be convicted of treason since the 1980s.
Lee was found guilty of leading a secret group of 130 members with alleged links to North Korea.
Tension in the Korean peninsula escalated in early 2013, when Pyongyang declared an armistice agreement between the two Koreas invalid and said it could carry out nuclear strikes against its southern neighbor.
The court was told that Lee thought that the heightened tensions could lead to war between the North and South, and he allegedly directed members of the group to attack major national facilities in the event of such a conflict.
"Piecing together the statements from the informants and evidences ... the defendants are found guilty for conspiring to start riots aimed at subverting the constitution," Suwon District Court said in a statement.
It said a strong penalty was inevitable because Lee's plot "posed substantial and clear threats to the existence of Korea and order of liberal democracy."
Under South Korean criminal law, a crime of rebellion can be committed when more than two people gather to conspire treason.
Lee denied all charges against him, while his party, the Unified Progressive Party, described the case as "a medieval witch-hunt."
Some local media and analysts said the investigation was being used to divert attention from allegations that South Korea's state spy agency - the National Intelligence Service - had been involved in manipulating South Korea's 2012 presidential election.