SEOUL, South Korea -- South Korean prosecutors indicted a left-wing lawmaker Thursday on charges that he was plotting a pro-North Korea rebellion to overthrow the government, saying his plan posed a "grave" national security threat.

Lee Seok-ki, from the small United Progressive Party, was arrested by South Korea's spy agency earlier this month for allegedly discussing launching strikes on national infrastructure with his colleagues in May in the event of a war with North Korea. The National Intelligence Service later handed him over to prosecutors.

Lee has flatly denied the allegation, saying the spy service fabricated the charges to divert criticism that its agents allegedly posted online messages supporting the conservative ruling party candidate and now-President Park Geun-hye and smearing her main liberal rival ahead of December's presidential election.

The case triggered a massive political and media firestorm in South Korea, with critics raising questions over whether there is any substantial evidence to back up Lee's alleged rebellion plot and noting past military-backed authoritarian governments often used rebellion charges to suppress political rivals.

North Korea angrily reacted to the scandal, calling it a "witch hunt" targeting those espousing greater reconciliation on the Korean Peninsula. North Korea last week cited the rebellion plot as an example of South Korea's confrontational postures that forced it to cancel this week's planned reunions of families separated by the 1950-53 Korean War.

Senior prosecutor Kim Soo-Nam told a news conference Thursday that Lee and his colleagues specifically brought up possible targets to attack, including a telecommunications facility in Seoul, during the May meeting, which drew 130 people. He said the plotters also discussed using websites to find ways to manufacture firearms and bombs. According to transcripts of conversations at the meeting publicized by South Korean media, some participants talked about ways to make more powerful BB guns and searching the Internet to find ways to build homemade bombs.

Kim said Lee believed that high tensions between the two Koreas this past spring would lead to war.

"It's an incident that an underground revolutionary organization ... systemically and collectively plotted to overthrow a free democracy and posed a grave threat to" South Korea's national security, Kim said.

He said Lee was indicted on two others charges -- inciting a rebellion and praising North Korea in violation of South Korea's anti-Pyongyang security law. Three of Lee's colleagues were also indicted, he said.

Defence lawyers for Lee said in a statement that prosecutors still lack decisive evidence to back up their indictment.

Earlier this year, North Korea sharply raised tensions by threatening nuclear war in anger over toughened UN sanctions over its third nuclear test in February. Animosities have gradually eased since then.