Prosecutors said Thursday they have indicted a senior member of a pro-Pyongyang activist group and his colleague after concluding they colluded prior to the former's unauthorized trip to North Korea earlier this year.

Earlier last month, the No. 2 official Ro Su-hui of the Pan-National Alliance for Korea's Reunification in Seoul was arrested soon after his return from an illegal visit to the North, during which he attended a memorial service for Pyongyang's former leader Kim Jong-il and took part in various propaganda events.

Prosecutors later arrested the secretary general of the organization, Won Jin-wook, on Tuesday and pressed charges against him for reportedly masterminding Ro's 104-day trip and colluding with North Korean spies from the Pyongyang branch of the organization, according to prosecution officials.

"We have confirmed that Ro's illegal entry into the North was not a one-man offense but was actually co-arranged by the group's offices in both Koreas," officials said.

South Korea's National Security Law prohibits citizens from visiting the North without prior permission and from praising the North. (Yonhap News)