Seoul, South Korea - A minority leftist political party that was constitutionally dissolved last month has said it will fight in court to maintain its parliamentary seats, and accused the conservative government of authoritarian tendencies.

Although there is no constitutional way of resurrecting the party, its legislators have pledged to file a petition at the Seoul Administrative Court on Tuesday to challenge the Constitutional Court's right to de-seat them.

"Korean society is being overwhelmed by Korean-style McCarthyism!" Oh Byeon-yoon, a former Unified Progressive Party legislators, said angrily on Monday.

The UPP was banned on December 19 by the Constitutional Court, which found UPP members to have attended assemblies to discuss North Korea-related insurrection.

The court also ordered the party's five representatives to forfeit their parliamentary seats.

As a legal defence against North Korea-inspired dissidents, South Korea maintains a wide-ranging National Security Law, which some believe is outdated and draconian.

The court launched its probe into the UPP at the request of the conservative Park Geun-hye government. It is the first time since Korea's 1987 democratisation that a party has been banned.

In a sign of the furious emotions surrounding the case, the central Seoul press conference location was besieged by crowds of conservative protesters, largely retired males wearing military fatigues, and busloads of riot police.

The Constitutional Court wrote that the "unique situation" of South Korea, facing "confrontation" with North Korea, merited consideration.

Lee Jae-hwa, the UPP's legal counsel, interpreted the court's statement to mean " that democracy is not based on pluralism, but on anti-communism".

Amnesty International, the UK-based human-rights organisation, and the US-based Carter Center have expressed misgivings over the case.