Prosecutors on Monday sought a 20-year prison term for a leftist lawmaker accused of plotting to overthrow the government.

Rep. Lee Seok-ki of the Unified Progressive Party is standing trial on charges of leading a clandestine organization, Revolutionary Organization, and plotting to overthrow the South Korean government in the event of an inter-Korean war, as well as sympathizing with North Korea, in violation of the South's anti-communist National Security Law.

Prosecutors also demanded that Lee's rights, such as suffrage, be stripped for 10 years following his release from prison.

The 52-year-old sitting legislator allegedly talked with about 130 other RO members during late-night secret meetings in central Seoul about blowing up key infrastructure in South Korea, including communication lines and railways, prosecutors said.

Prosecutors further alleged Lee made remarks sympathetic to North Korea and sang its military march songs before hundreds of members who attended the organization's meetings held between March and August of last year, according to prosecutors. Such acts are in violation of the security law, prosecutors added.

"Heavy punishment is necessary since (Lee) ordered RO members to prepare for riots and military warfare, by abusing his status (as a lawmaker)," prosecutors said during a trial at the Suwon District Court in Suwon, just south of Seoul.

"The only way to prevent the recurrence of similar crimes is putting Lee behind bars for a long period of time," prosecutors argued, citing his prior conviction for violating Seoul's anti-communist law. 

Prosecutors also demanded 15-year prison terms for five key UPP members and a 10-year term for another UPP member.

A sentencing trial for Lee and the other six is scheduled to be held within two weeks, court officials said.

If Lee is convicted, it would be the first time in nearly 30 years that someone is put behind bars for plotting an insurgency to overthrow the government.

Past military-backed governments used insurgency charges to suppress political dissidents. However, there have been no indictments over insurgency plots or other similar anti-government charges in recent years since democratically-elected leaders replaced the country’s past military dictators.

Meanwhile, the National Assembly is currently reviewing a motion introduced by the ruling Saenuri Party that calls for expelling Lee. His expulsion is subject to two-thirds approval from the ethics committee and then two-thirds approval from the full National Assembly.

Also, the Constitutional Court is currently reviewing a petition filed by the government calling for the disbandment of the UPP. (Yonhap News)