Japan held an annual rally on Friday over Tokyo's claim to a set of tiny islands controlled by South Korea, which has been at the centre of a long-standing territorial feud.
Some 500 people flocked to the event in Shimane prefecture in western Japan, including the highest-ranking Japanese government official ever to attend and local and national politicians.
Seoul reacted angrily to the rally and the expected presence of Aiko Shimajiri, a parliamentary secretary in Tokyo's Cabinet Office, with foreign ministry spokesman Cho Tai-Young describing it as "greatly regrettable".
"Japan should not conduct such behaviours (if it hopes) to promote friendship between Seoul and Tokyo," he added before the ceremony began, warning of "appropriate measures".
Tokyo refers to the islands in the Sea of Japan (East Sea) as Takeshima while they are known as Dokdo in South Korea.
"Takeshima is an integral part of our country," Chief Cabinet Secretary Yoshihide Suga told a news conference in Tokyo.
"We dispatched our parliamentary secretary as a matter of course."
Police officers were deployed to guard the Shimane Prefectural Assembly Hall, the venue of the rally, as protesters from both countries were expected to stage separate demonstrations outside.
Relations between the two countries have regularly been strained by the territorial dispute and other issues of contention arising from Japan's 1910-45 colonial rule over the Korean peninsula.
The territorial row deteriorated last year following a surprise visit by South Korean President Lee Myung-Bak to the island chain, as Tokyo remains embroiled in separate territorial spats with China and Russia.
Japan's conservative Prime Minister Shinzo Abe, who swept national elections in December, sent a message to South Korean president-elect Park Geun-Hye last month seeking a new start to a relationship dogged by bitter historical disputes.
During his past stint as prime minister, Abe angered South Koreans by denying the Japanese military's direct involvement in forcing women into sexual slavery during World War II.