TOKYO -- North Korea angrily denounced the arrest of 23 of its supporters by South Korean security agents and said the detentions cast a shadow over upcoming North-South summit talks, Radio Pyongyang said Wednesday. South Korea's Agency for National Security Planning on Sunday arrested 23 South Koreans suspected of having actively supported the North, in violation of a ban on all contacts with the hardline communist state. The agency said the 23 were members of the National Salvation Advance Guard, a pro-North Korean underground group. North Korea's Committee for the Peaceful Reunification of the Fatherland called the incident an anti-North plot fabricated by Seoul, said Radio Pyongyang, monitored in Tokyo.