In an unusually speedy manner, the nation's spy agency on Wednesday arrested a leftist lawmaker on charges of plotting to overthrow the government, putting it a step closer to launching a prosecution probe into the scandal.

Rep. Lee Seok-ki of the minor opposition Unified Progressive Party (UPP) is accused of leading a secret organization with suspected links to North Korea. The so-called Revolutionary Organization (RO) allegedly devised plans to destroy key infrastructure facilities in the South in the event of an inter-Korean war, including communication lines and railways.

The arrest was made shortly after the Suwon District Court issued a warrant against Lee. National Intelligence Service (NIS) investigators brought him to a police station in Suwon, south of Seoul, where he was taken into custody.

Lee will undergo a warrant examination on his arrest at the Suwon court Thursday.

The NIS carried out the arrest prior to the warrant review due to concern over Lee being a flight risk.

In executing the warrant at Lee's office at the National Assembly, NIS investigators had nearly an hour-long confrontation with vehemently-resisting UPP members who blocked the path to Lee's office.

He later came out of his office voluntarily and complied with the investigators.

Just a few hours earlier, the Assembly passed a motion for Lee's arrest in a secret vote at a plenary session by an overwhelming margin of 258-14. Eleven abstained, while six other votes were deemed invalid.

Lee and his party have denounced the scandal as a "witch hunt" and fiercely protested against bringing the arrest motion to the floor.

"It would be an indelible mistake for the National Assembly to go along with a fabricated rebellion conspiracy, which (I) will be acquitted of in just a few months time," Lee said in his address to the Assembly ahead of the vote.

Lee's detention was subject to parliamentary consent, as lawmakers in South Korea are immune from arrest while the Assembly is in session.

The motion was filed with the Assembly on Monday following the prosecution's request last week for a warrant to detain Lee for questioning.

The lawmaker, who had previously been jailed for pro-North Korea activities, was elected to the Assembly on a proportional representation ticket in last year's legislative elections.

Lee's party has claimed the investigation is a fabrication aimed at diverting public attention from allegations that the National Intelligence Service meddled in last year's presidential election. That scandal centers on allegations that the spy agency conducted an online smear campaign to sway public opinion in favor of the ruling party.

According to the motion, Lee held a secret meeting with RO members in May, during which he allegedly called for "simultaneous preparations en masse for war," saying North Korea nullified the 1953 armistice that halted fighting in the 1950-53 Korean War.

The communist nation declared the Armistice Agreement null and void in March as the regime stepped up war rhetoric against South Korea and the United States in anger over American-involved annual military exercises.

Lee also said that the National Assembly is a "bridgehead" for "revolutionary struggle" and urged the members to regard their workplaces as the "most front-line guard posts," according to the motion.

The RO has more than 130 members, it added. (Yonhap News)