China on Tuesday launched a counter-protest against Japan after Hong Kong-based activists were prevented from landing on islands claimed by both countries.
"The Japanese action against the Chinese activists violates international law and infringes on Chinese sovereignty," foreign ministry spokesman Liu Jianchao told reporters.
"It is illegitimate, and China is strongly opposed to that."
The four activists sailed into the contested area on Sunday flying a Chinese flag and a banner claiming sovereignty over the uninhabited islands in the East China Sea, which are known as Diaoyu in Chinese and the Senkaku in Japanese.
The Japan Coast Guard dispatched patrol ships and an aircraft to chase away the activists, who never reached the islands but spent more than an hour in what Japan considers its waters, Japanese coast guard officials said.
The coast guard said it gave up the chase after the vessel changed direction, and Japan immediately lodged a protest with China over the activists' actions.
"It is a serious problem that they infringed on our nation's sovereignty, even for a short period of time," Japanese chief government spokesman Nobutaka Machimura said.
"There is no doubt historically and under international law that the Senkaku islands are Japan's sovereign territory. No territorial dispute exists between the two countries."
Japan and China, Asia's two largest economies, have a series of territorial disputes, including over lucrative gas resources in the East China Sea.
Japan claimed the islands in 1895 when it took over Taiwan.
The United States seized the islands in World War II and returned them to Japan in 1972 along with Okinawa, despite protests from China and Taiwan. China also claims Taiwan as a whole.
In March 2004, seven Chinese activists managed to land on the islands. They were arrested and deported, causing a diplomatic row with Beijing.
