South Korean police have raided the office of a pro-North Korean activist group as part of a probe into its alleged violation of the nation's draconian security law, officers said Wednesday.

The so-called "sopoong" is a Seoul-based organization formed in 2006 by pro-North activists. The group claims its goal is to call for the reunification of the two Koreas.

The Seoul Metropolitan Police Agency (SMPA) said it has also searched the houses of 10 members of the organization to search and seize relevant digital files and documents.

Lee Joon-il, the chairman of the minor opposition Unified Progressive Party's Jungrang district, has also been arrested in connection with the probe, the SMPA said. Lee previously served as the group's chief.

South Korea's National Security Law prohibits citizens from forming an organization that praises, encourages or propagandizes the communist North.

"We plan to summon the persons concerned after analyzing seized documents, speech notes and digital files," an SMPA officer close to the investigation said.

The security law has long been politically controversial.

Conservatives claim the law is necessary to maintain public order amid North Korea's efforts to spread its communism to the South, while liberals say the law is outdated and often used as a tool to oppress dissidents and limit freedom of expression.

The United Nations and human rights groups have called on Seoul for years to repeal or revise the law, which the country's past military dictators had abused by using it against not only people suspected of being spies but also against political dissidents.

South and North Korea remain technically in a state of war since the 1950-53 Korean War ended in an armistice, not a peace treaty. (Yonhap News)