SEOUL, South Korea — A South Korean court sentenced an opposition lawmaker to 12 years in prison on Monday for forming a “revolutionary organization” and conspiring to start an armed revolt to overthrow the Seoul government in the event of war with North Korea.

Lee Seok-ki, a politician affiliated with the far-left United Progressive Party, became the first South Korean lawmaker convicted on charges of plotting treason since the country’s past military dictators used them to silence dissidents decades ago.

The arrest of Mr. Lee, 51, in September and his subsequent court hearings drew intense public attention in South Korea, where an ideological conflict rooted in fear of the Communist North shows no sign of easing more than 60 years after the end of the Korean War in 1953.

Mr. Lee has denied the charges against him, and his lawyers said on Monday that they would appeal.

In recent months, conservative politicians, news media and activists have become increasingly vocal in criticizing “jongbuk,” or followers of North Korea, accusing them of infiltrating society and spreading subversive ideas. Meanwhile, members of the progressive opposition have accused the conservative government of President Park Geun-hye of resorting to a witch hunt to repress her political enemies.