The Foreign Ministry said on Friday that the landing of two Japanese citizens on the Diaoyu Islands has seriously violated China's territorial sovereignty.
A Japanese coast guard official said on Friday that two Japanese nationals landed on the islands on Thursday, despite warnings from the coast guard to stay away.
"Despite our warnings, they dived into the sea from a fishing boat and reached the islands," the official said, adding that the two stayed for 90 minutes before leaving.
Media reports said the two Japanese right-wing activists went ashore on Beixiaodao, an islet of the Diaoyu Islands.
One of the two people who landed on the islands was Hitoshi Nakama, a local politician from Ishigaki, Okinawa. He also landed on the islands in January.
The case is under investigation by both the coast guard and police, the official said.
"The illegal acts of the Japanese right-wingers have seriously violated China's territorial sovereignty. China has lodged a solemn representation and protest to the Japanese side," Foreign Ministry spokesman Liu Weimin said at the ministry's regular news conference.
"The Diaoyu Islands and its affiliated islands have been Chinese territory since ancient times and China has indisputable sovereignty over the islands," Liu said.
The incident came one day after Taiwan vessels tangled with Japanese patrol ships in waters close to the islands and left a Chinese national flag on waters 10 meters from the shore.
"The Japanese move should be seen as a reaction to the visit of Taiwan vessels to the Diaoyu Islands," said Zhou Yongsheng, a professor of Japanese studies at China Foreign Affairs University in Beijing.
Ties between Beijing and Tokyo were tested in the autumn of 2010 when a Chinese trawler collided with patrol boats from the Japanese coast guard in waters near the islands. China later froze bilateral high-level exchanges, including some military ones.
The hostility was re-ignited again this year when Governor of Tokyo Shintaro Ishihara said in April that his city prefecture was negotiating with the "owner" of the islands in the hope of "buying them by the end of this year" and started raising money for the proposal.
Also on Friday, Ishigaki Mayor Yoshitaka Nakayama visited Tokyo to meet Ishihara and sent the governor documents, including data on the local ecological system of the islands, expecting him to "make full use" of the materials to claim them.
"Apparently Nakama's deeds are well arranged and he has acted in concert with the mayor," Zhou said.
Former high-level military officers and prominent scholars from China and Japan earlier this week called on Beijing and Tokyo to stop the "endless" dispute over the islands at a non-governmental meeting on bilateral ties.
The Foreign Ministry on Friday also responded to reports that the Republic of Korea has postponed the submission of a claim to an extended portion of continental shelf in the East China Sea to a United Nations commission until sometime "within this year".
The ROK is expected to claim that the Korean Peninsula's naturally extended continental shelf stretches to the Okinawa Trough in the East China Sea, a move deemed certain to rekindle a territorial dispute with Japan and China over the extent of their respective exclusive economic zones.
"China's claim in the East China Sea is clear and consistent, and the Republic of Korea is clear about that," Liu Weimin said.
"As for the disputes in the waters, the Chinese side would like to have them settled through negotiations with concerned countries."