A left-wing South Korean activist was detained Thursday when he returned home from an illegal trip to North Korea and a meeting with its ceremonial head of state, police said.
Security officials held Ro Su-Hui soon after he walked across the border at the truce village of Panmunjom. Seoul says he faces stern punishment for his trip, during which he praised Pyongyang's rulers.
Ro, 68, went to Pyongyang on March 24 without Seoul government approval for a memorial service marking the 100th day since the death of longtime ruler Kim Jong-Il.
More than 1,000 conservative activists rallied with anti-Pyongyang placards at Paju near a border bridge leading to Panmunjom, demanding he be punished.
They torched a North Korean flag and an effigy of current leader Kim Jong-Un along with his portraits.
"Expel traitor Ro Su-Hui to North Korea!" one placard read.
Pictures provided by the South's unification ministry showed about 300 North Koreans waving flags on the northern side of Panmunjom as Ro, wearing a grey suit, crossed the frontier watched by border guards.
The North Koreans gave him "a hearty send-off", shouting slogans such as "Down with the (President) Lee Myung-Bak group of traitors!", according to the North's official news agency KCNA.
He was taken to a police station at Paju, police said.
The detention came hours after police raided Ro's home and office in Seoul to seize computer hard discs and documents.
He could face up to 10 years in prison under a tough security law that penalises pro-Pyongyang activity and bans citizens from going to the North without approval.
Ro is a member of pro-Pyongyang group the North and South headquarters of the Pan-national Alliance for Korea's Reunification, which said the visit was justified.
It accused Seoul of trying to use his return to eliminate "progressive pro-reform forces" before presidential elections in December, and called Ro's visit "a token of due courtesy" for North Koreans during the mourning period for the late leader.
In an interview Tuesday with the North's official news agency, Ro praised Jong-Un for "pursuing politics of love for the people and posterity".
"In the North the people are regarded as Heaven, children as kings and the single-minded unity is stronger than nuclear weapons," he said.
He met the ceremonial head of state Kim Yong-Nam in Pyongyang Wednesday.
Seoul did not send any representatives to Kim's funeral and approved a trip only by two private delegations, a decision strongly criticised by Pyongyang.
The North has ratcheted up hostility towards Seoul for perceived insults to the late leader during the mourning period and other alleged slights.
Other activists visiting the North have been charged or jailed on return. Han Sang-Ryol, a pastor, was sentenced to three years in jail last year for an unauthorised trip in 2010.