SEOUL (REUTERS) - South Korea's Constitutional Court ruled on Friday to disband a minority left-wing party accused of pro-North Korea activities that it said violated the basic principles of free democracy and stripped its five members of parliament of their seats.

Eight of the court's nine judges sided with the majority opinion for the unprecedented decision that "there was an urgent need to remove the threat posed by the party to the basic order of democracy," the chief judge Park Han Chul said.

The case against the United Progressive Party (UPP) brought by the government of President Park Geun Hye has been played out amid a bitter struggle between conservatives and liberals in a country sharply divided over ideology and North Korea.

The ruling to disband a political party required the votes of six of the nine judges.

UPP leader Lee Jung Hee said after the ruling that the court had "opened the gate to totalitarianism based on falsehood and imagination" and effectively turned South Korea into "a dictatorial country."

The party, which says it speaks for the working class and progressive members of society, has led a stormy existence since its founding in 2011, including the conviction of one member of parliament earlier this year on charges of treason.

Lee Seok Ki awaits the Supreme Court ruling on his appeal in a case stemming from his comments at a party meeting last year where he reportedly called for attacks against South Korea's utilities if a war between the rival Koreas breaks out.

Lee has pleaded innocent, saying his comments were taken out of context and misinterpreted.

Justice Minister Hwang Kyo Ahn, who was the plaintiff in the Constitutional Court case, has argued that the UPP has sympathised and followed North Korea's strategy to revolutionise the South.

"What the so-called 'progressive democratic movement' is pursuing is the establishment of a pro-communist government and unification to realise North Korean-style socialism," Hwang told the court in the final argument last month.

Amnesty International expressed concern at what it said was the use of national security to repress political opposition.

"The ban on the UPP raises serious questions as to the authorities' commitment to freedom of expression and association," Roseann Rife, East Asia Research Director at Amnesty International, said in a statement.