For the first time a sitting member of the National Assembly is under investigation for plotting a rebellion in Korea.
Lawmaker Lee Seok-ki of the ultra-left Unified Progressive Party and 13 other left-leaning figures are accused of setting up a secret cell to overthrow the state.
The National Intelligence Service has slapped a travel ban on the 14, and three officials in Lee's party were earlier arrested on sedition charges.
Lee, who allegedly helmed the conspiracy, on Thursday denied the charges.
In a meeting of party leaders, the lawmaker said the NIS, which has been criticized for meddling in last year's presidential election, fabricated the case as part of an orchestrated campaign against left-leaning politicians.