Four opposition lawmakers pleaded not guilty on Monday at the first hearing of a trial over their alleged illegal detainment of a spy agent ahead of the 2012 presidential election.

On Dec. 11, 2012, four members of the now-defunct Democratic Party set up camp outside the home of a National Intelligence Service (NIS) agent, identified only as a woman surnamed Kim. She had allegedly posted politically sensitive messages online ahead of the presidential election scheduled for later that month.

The four lawmakers, who are now members of the main opposition New Politics Alliance for Democracy, said the 35-hour campout did not constitute illegal confinement.

"We did not confine Kim. We were only trying to uncover the truth behind the NIS's illegal electioneering," an attorney said on behalf of the four at the hearing held at the Seoul Central District Court.

South Korea's Public Official Election Act prohibits NIS officials from voicing their political opinions ahead of the presidential election.

While agent Kim was never charged with violating the act, two former ranking NIS officials and the former head of the NIS, Won Sei-hoon, were found guilty of illegal electioneering by an appellate court last month. Won has since requested a Supreme Court trial.

The election victory of President Park Geun-hye, the then ruling party candidate, over her main opposition candidate came at a slim margin of 3.6 percentage points. The neck-and-neck result has unnerved opposition lawmakers, who claim the NIS's alleged online smear campaign against their candidates is what turned the tide in Park's favor.

"Even if what we did comprises illegal confinement, the fact that the NIS meddled in the election should exempt us from punishment," the lawmakers' attorney said.

The second hearing is scheduled for March 23. (Yonhap)