SEOUL – South Korean prosecutors indicted a leftwing 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.

Former lawmaker Lee Seok-ki, from the small United Progressive Party, was arrested by South Korea’s spy agency this month for allegedly discussing launching strikes on national infrastructure in South Korea 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 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.

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,” Kim said.

Three of Lee’s colleagues were also indicted Wednesday over the alleged rebellion plot, Kim said.