SEOUL (AFP) - A South Korean court on Monday sentenced an opposition legislator to 12 years in prison after a rare treason trial saw him convicted of plotting an armed revolt in support of North Korea.

Prosecutors had demanded 20 years for Lee Seok Ki, 52, who was tried along with six other members of his left-wing United Progressive Party.

Lee was the first member of the National Assembly to face treason charges since the country's transformation from a military-backed autocracy to a fully-fledged democracy in the 1980s.

As well as his prison term, the court ordered Lee deprived of his civil rights for 10 years following his eventual release. After parliament voted to lift his immunity from arrest, Lee was charged last September under the 65-year-old National Security Law, which rights groups have accused past administrations of using to stifle debate and silence political opposition.

The charges related to meetings Lee held with his supporters in May last year, at a time of surging military tensions following the North's third nuclear test.

The court was played tapes of Lee telling members of his group to prepare attacks on South Korea's communication lines and railways in case of a full-scale conflict breaking out with the North.

"We see sufficient evidence that (the defendant) plotted a revolt and planned collective actions to carry it out," said the court ruling.

Lee steadfastly denied all the charges, saying he was the victim of a "witch hunt" by South Korea's spy agency aimed at deflecting public attention from a scandal involving a number of its agents meddling in the 2012 presidential election.

Lee has been in trouble for his political views before. In 2002, he was sentenced to two-and-a-half years for anti-government activities. He received a presidential pardon later the same year.