SEOUL, South Korea — A South Korean activist was arrested on Thursday after he returned from an unauthorized visit to North Korea, where he called for the reunification of the two Koreas and bitterly criticized President Lee Myung-bak of South Korea for his hard-line North Korea policy.

The activist, Ro Su-hui, 68, entered North Korea on March 24 through China to observe the 100-day mark after the death of Kim Jong-il, the North Korean leader, which he called “the greatest sorrow of the Korean nation,” according to the North’s state-run media. But he chose to return home by walking across the heavily guarded demilitarized zone separating the two Koreas.

Defying South Korea’s anti-communist National Security Law, seven other activists since the late 1980s have made similar unauthorized visits, in a gesture that they said symbolized their wish for Korean unification. The law bans sympathizing with North Korea and punishes unauthorized visits like Mr. Ro’s with up to 10 years in prison.

Such trips touch raw emotions in South Korea, where the desire for reunification with North Korea coexists with a deep hatred and fear of the totalitarian dictatorship in Pyongyang.