SEOUL • A former director of South Korea's main spy agency was sentenced to four years in prison yesterday on charges of ordering a clandestine online campaign to help the 2012 election of former president Park Geun Hye, who has since been impeached.

Won Sei Hoon, director of the National Intelligence Service (NIS) from 2009 to 2013, was convicted of violating laws that bar public servants from campaigning for political candidates and prohibit the powerful spy agency from interfering in domestic politics.

After the ruling yesterday at the appeals court in Seoul, Justice Kim Dae Woong had Won sent directly to a detention centre from the courtroom.

In the run-up to the December 2012 election, Won directed a secret team of psychological warfare agents to work with hired private bloggers on an online smear campaign against Park's political rivals, according to the prosecutors' indictment.

In numerous posts on Twitter and other online forums, the agents tried to sway public opinion in favour of Park, praising her as "the only answer" to North Korean threats, prosecutors said.

The agents ridiculed her enemies as pro-North sympathisers.

They called her main rival, then opposition leader Moon Jae In, "childish" and "an ex-convict" for being arrested for protesting against the military dictatorship that ruled South Korea in the 1970s, according to the prosecutors.

Park defeated Mr Moon by a margin of 3.5 percentage points, or roughly a million votes.

But Park was impeached by the National Assembly in December - and was formally removed from office in March - on corruption charges after months of huge anti-government protests in downtown Seoul.

Mr Moon won the election to replace her in May.

"It's unprecedented for a state agency to have intervened in politics and elections (for) so long and so systematically," Justice Kim said in his ruling, referring to the NIS.

Won had violated the political neutrality of the NIS by "spreading pro-government opinions through online postings", the Seoul High Court said in its ruling.

The spy agency has repeatedly pledged not to abuse its power.

But scandals at the secretive agency have frequently rocked the country, with several of its former directors imprisoned on corruption and other charges.

Won was first indicted in 2013 while Park was still in power.

In 2015, he was sentenced to three years in prison and was arrested.

But later that year, the Supreme Court rejected some of the evidence used against him and ordered a lower court to review the case.

Won was released on parole.

Yesterday, the appeals court again convicted Won after accepting more incriminating evidence presented by prosecutors.

During the court hearings, Won called his agency's Internet activities a legitimate operation to guard South Korea against the North's influence.

His lawyers said they would fight the verdict again at the Supreme Court.

NEW YORK TIMES, AGENCE FRANCE-PRESS