Hong Kong (CNN) -- A diplomatic row has erupted between Japan and China after a group of Chinese nationals were photographed raising flags on a disputed island in the East China Sea before being arrested, the latest incident in rising territorial tensions in North Asia.
Japanese authorities on Wednesday arrested 14 Chinese activists at the disputed islands.
"We will deal with the issue sternly based on the law," Japanese Prime Minister Yoshihiko Noda told reporters Wednesday night, as Japan's foreign ministry summoned the Chinese ambassador to protest strongly what it called an illegal landing.
In a press conference, Japanese Chief Cabinet Secretary Osamu Fujimura said, "Senkaku Islands are our sovereign territory historically and in the international law. We are controlling the islands effectively." He said the activists were given repeated warnings and that their landing was "very regrettable."
China says it has sovereignty over the East China Sea islands, which it calls Diaoyu, and has urged Japan to immediately release the detained citizens.
On Wednesday Vice Foreign Minister Fu Ying summoned the Japanese ambassador and phoned her Japanese counterpart to "lodge solemn representation over Japan's illegal arrest," according to the Chinese Foreign Ministry.
In Hong Kong, Chief Executive C.Y. Leung summoned Japanese Consul-General Yuji Kumamaru, expressing similar "extreme" concern. Hong Kong is a special administrative region of China.
Five of the men who landed on the islands were taken into custody by Okinawa police, while the nine others on the vessel were detained by the Japan Coast Guard.
At least six are from Hong Kong, including former Legislative Council member "Bull" Tsang Kin-shing, according to the Hong Kong Immigration Department, which on Thursday sent two officers to Okinawa, Japan, where the group is being detained. Leading the activists is the Hong Kong-based Action Committee for Defending the Diaoyu Islands.
In addition to Tsang, the group included a labor union chairman, a teacher, two TV journalists and seven crew members, according to lists provided by the police and coast guard. The Hong Kong-registered fishing vessel, which had set sail on Sunday, was detained and the men charged with violating the immigration control and refugee recognition act, according to Japanese and Hong Kong authorities.
According to the Hong Kong group's Twitter account, they planned to demolish the Japanese lighthouse on the island, sing the national anthem and set up a television and radio to receive Chinese broadcasts.
A tweet three hours later reassured readers of their safety and eventual return after being "invited to stay in Okinawa overnight to have sushi and ramen."
A picture of the activists showed a Taiwan flag being borne; it was not clear whether the self-identified Chinese nationals included anyone from Taiwan, which China considers a renegade province.
The arrests have led to anti-Japanese protests in Shanghai, Hong Kong and Beijing.
The Wednesday incident coincided with the 67th anniversary of Japan's official World War II surrender. On the same day, two Japanese Cabinet ministers visited the controversial Yasukuni shrine in Tokyo, which honors Japan's war dead as well as war criminals.
China and South Korea, given their respective wartime occupation and colonization by Japan, have condemned such visits.
Adding to the regional tensions before the anniversary was South Korean President Lee Myung-bak's visit to what the country calls Dokdo, a small group of islets that Japan claims as Takeshima. The move prompted Japan to recall its ambassador to Seoul.
On Tuesday, Lee said in remarks to teachers that Japanese Emperor Akihito should make a "sincere apology" for suffering under Japanese colonial rule.
The following day, amateur South Korean athletes, including the rock singer Kim Jang-hoon, swam in a 49-hour, 220-kilometer relay toward the island to mark the anniversary of their country's independence from Japan in 1945.
CNN's Steven Jiang, Alexis Lai, Junko Ogura and Yoko Wakatsuki contributed to this report.