South Korea arrests activist after he visits North Korea

(CNN) -- A South Korean left-wing activist was arrested Thursday for an unauthorized visit to North Korea, a South Korean national police official said.

Roh Su-hui, a leader with an organization close to North Korea, was taken into custody as he crossed back into South Korea after three months, the official said.

Roh visited Pyongyang to commemorate the 100th day since the death of the country's late leader, Kim Jong Il, according to KCNA, North Korea's official news agency. KCNA called his visit "a righteous deed in view of traditional beautiful manners" and urged the South not to arrest him.

Nevertheless, he was immediately arrested for illegal entry and glorifying his country's political rival.

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He walked across the two countries' heavily guarded border in the demilitarized zone and was brought in for questioning, according to an official from the South's National Police Agency who refused to be named in line with Korean policy.

So far, he has refused to speak about his visit, the official said. Authorities will be interrogating him and will likely charge him under the National Security Law (NSL) within two days, the official told CNN.

The NSL was designed to protect the state against North Korean propaganda and to prosecute spies. It was used widely during the years of military dictatorship in the 1970s and '80s.

"Roh Su-hui's illegal entry to North Korea and his slandering of the South's government and praising of the North is an illegal act. Upon Roh Su-hui's return, he will be investigated on the purpose of his visit and his whereabouts in the North and will be strictly dealt with in according to relevant law," Unification Ministry spokeswoman Park Soo-jin said in a regular briefing Wednesday.

North Korea claims that "the South is using Roh's return as a pretext for eliminating the progressive pro-reform forces."

South Korea did not send any representatives to the North after Kim's death. Tension between the two countries remains high since North Korea's shelling attack on a South Korean island in 2010 and the sinking of a South Korean warship in a torpedo attack blamed on the North.