Seventeen people, including three lawmakers of South Korea's main opposition Democratic Party, have landed on Sea of Japan islands also claimed by Tokyo.
Setting foot on the islands, called Takeshima in Japan and Dokdo in South Korea, on Tuesday, they criticized Japan for claiming sovereignty over the islands, which are under the effective control of South Korea, according to the party.
Japan's Foreign Ministry lodged a protest with the South Korean side later in the day.
Hiroyuki Namazu, director-general of the ministry's Asian and Oceanian Affairs Bureau, told Kim Jang-hyun, a senior official at the South Korean Embassy in Tokyo, that the islands are obviously the inherent territory of Japan in light of historical facts and based on international law.
The landing was totally unacceptable and extremely regrettable, Namazu said.