A vocal North Korean defector and activist known for flying anti-Pyongyang leaflets into the North by balloon was questioned by police on Monday over his recent claim to have sent leaflets, in the first violation of South Korea's anti-leaflet law that took effect in March.

Park Sang-hak, the head of Fighters for a Free North Korea, a North Korean defectors' group, arrived at the Seoul Metropolitan Police Agency around 2 p.m. to be grilled as a suspect about his April 30 claim that his organization had sent a total of 10 balloons carrying around 500,000 leaflets, 500 booklets and 5,000 $1 bills towards the North on two occasions earlier that week.

He defended his activities as an effort to deliver the truth about South Korean society, politics and free democracy to the North Korean people.

Under the revised Development of Inter-Korean Relations Act, which took effect in March, the flying of propaganda leaflets across the border is banned and violators can be subject to a maximum prison term of three years or a fine of up to 30 million won ($26,930).

Park's alleged leafleting in April could be the first violation of the revised law.

The police launched an investigation into the case and raided Park's office in Seoul on Thursday after the sister of North Korean leader Kim Jong-un slammed Seoul in early May for failing to stop anti-Pyongyang leaflets flown by the defector group, calling it an "intolerable provocation" and warning of "corresponding action."

President Moon Jae-in indirectly mentioned the issue of anti-Pyongyang leaflets during his news conference on Monday by saying that it is never desirable to dampen inter-Korean relations by violating inter-Korean agreements and current laws.

Before entering the Seoul police agency, Park reacted to Moon's remarks, saying North Korea has to be changed and his colleagues will continue to send leaflets to the North even if he is imprisoned.

"Is it so wrong to let North Koreans know about Seoul's politics, culture and society, liberal democracy and the Universal Declaration of Human Rights?" he asked.

"It is North Korea who should change (its attitude)," Park said, referring to Pyongyang's act of exploding an inter-Korean liaison office in the border city of Kaesong last year in anger over similar leafleting.

Park's group has engaged in leafleting more than 60 times since the Ministry of Unification began keeping records in 2010. (Yonhap)