Tokyo lodged a protest with China on Saturday after spotting 230 Chinese vessels swarming waters near the Japan-controlled Senkaku Islands in the East China Sea, an unusually large number, the Foreign Ministry said.
The roughly 230 fishing boats and six China Coast Guard vessels were confirmed in a contiguous zone near the uninhabited islets, which are also claimed by China.
The number of fishing vessels, which appeared to be escorted by the coast guard ships, was larger than has previously been spotted.
"This is a unilateral act that raises tensions ... and it is unacceptable to us," the Foreign Ministry's Asian and Oceanian Affairs Bureau director-general, Kenji Kanasugi, told the Chinese Embassy in Japan, the ministry said.
Some of the coast guard vessels appeared to be equipped with guns, according to the Japan Coast Guard.
The development followed a similar incident Friday when a total of eight Chinese fishing and coast guard vessels briefly entered Japanese territorial waters around the Senkakus, prompting the ministry to summon the Chinese ambassador to lodge a protest.
Vice Foreign Minister Shinsuke Sugiyama told Chinese Ambassador to Japan Cheng Yonghua on Friday that the coast guard vessels' entry into territorial waters around the islets and their activities there are a violation of Japan's sovereignty and totally unacceptable, the ministry said.
The coast guard vessels entered Japanese waters soon after the Chinese fishing ships did at around 1:30 p.m. Friday, and were then confirmed to be sailing near the fishing vessels, the ministry said.
While previous protests against entry by Chinese coast guard vessels were lodged at the director-general level, the latest protest was at a higher level.
The remote islets administered by Japan are claimed by Taiwan as well.