SEOUL (AFP) - South Korea's parliament on Wednesday approved the arrest of a leftist lawmaker accused of plotting an armed insurrection in support of Pyongyang.

In a rare show of cooperation, rival political parties joined forces to pass a motion allowing the arrest of sitting United Progress Party (UPP) legislator Lee Seok Ki on charges of sedition.

Of the 289 lawmakers who took part in the vote, 258 said yes, 14 said no, 11 abstained and six other votes were nullified.

Hundreds of police, including riot officers carrying shields, stood guard outside the National Assembly as the vote was under way, with riot vans forming a blockade across access roads and a water canon atop an armoured vehicle.

About 200 members of the minor opposition UPP staged a sit-down protest on the steps leading to the doors of the assembly building, chanting slogans accusing the National Intelligence Service (NIS) of fabricating the charges.

Justice Minister Hwang Kyo Ahn told lawmakers that in May, Lee - believing war with the North was imminent - told his secretive leftist group to prepare for attacks on South Korea's communication lines and railways.

He said he was the victim of a "savage and irrational witch hunt" led by the country's secret police and fanned by the conservative news media.

"They may jail me for a while but steps towards independence, peace and democracy will never falter," he told parliament.

As a lawmaker, Lee would ordinarily be immune from arrest while the assembly is in session, meaning his detention was subject to parliamentary consent.

His arrest was not expected immediately as parliament must first complete the necessary documentary procedures.

The request for arrest approval was made by the government, and the main opposition Democratic Party had earlier indicated it would support the ruling conservatives in passing the motion.

"We'll never tolerate anyone who is willing to fight on the side of the enemy in the event of a war," Mr Kim Han Gil, head of the Democratic Party, said.

The NIS last Wednesday raided UPP party offices and arrested three of Mr Lee's supporters on charges of seeking to instigate an armed insurrection in support of North Korea.

Lee reacted by describing the sedition charges as "sheer fabrication" and an attempt by the NIS to "block progressive and democratic forces".

Sedition charges have been extremely rare since South Korea introduced democratic elections in the late 1980s, and political analysts suggested the timing of the NIS action was likely to raise eyebrows.

A statement from the UPP denounced what it called a joint campaign by the presidential Blue House and the NIS to cloud the issue of an election-rigging scandal that has spawned large candle-light street protests in Seoul in recent weeks.

The scandal has seen the arrest of former NIS head Won Sei Hoon for allegedly ordering agents to run an online smear campaign against opposition presidential candidate Moon Jae In of the Democratic Party.

Mr Moon was narrowly beaten in the December poll by the ruling party candidate Park Geun Hye.

It is not the first time Lee has faced subversion charges.

He was arrested in 2002 and sentenced to two and a half years for working with an underground political party in the 1990s. He received a presidential pardon later in the same year.

It remains to be seen whether the NIS has secured solid evidence to make charges stand up in court.

The service, formerly known as the Korea Central Intelligence Agency, was tainted by many cases of human rights abuse under the country's former authoritarian government.

South Korea is still technically at war with the North, as the Korean War ended in a ceasefire in 1953 rather than a peace treaty.