Tokyo - Some 230 Chinese fishing vessels and seven coast guard ships, including four apparently carrying weapons, sailed into waters close to disputed East China Sea islands on Saturday (Aug 6), Japan's foreign ministry and coastguard said.
Six Chinese coast guard ships were spotted earlier in the day and late on Saturday afternoon.
Japanese officials said they spotted another, which seemed to be carrying arms, in the contiguous waters of the uninhabited islands.
The two countries are locked in a long-running dispute over the rocky islets - known as the Senkaku in Japan and the Diaoyu in China - but it is rare for so many Chinese fishing vessels to be spotted in the disputed waters.
"We cannot be sure about what the Chinese coast guards are doing for what purposes in the waters," a Japanese coast guard official told AFP.
The 230 fishing vessels and seven coast guard boats remained in the area nine hours after they were first spotted, he added.
The fishing vessels appeared to be engaged in operation, he said.
After catching sight of the coast guard ships in the contiguous waters at 8.05 am, the Japanese foreign ministry's Asian and Oceanian Affairs Bureau lodged a strong protest with the Chinese embassy in Tokyo, it said.
The foreign ministry, which also submitted a protest through its embassy in Beijing, demanded the vessels leave the disputed waters immediately and "never enter Japan's territorial waters", the ministry said in a statement.
"Japan can never accept activities by (Chinese) official vessels near the Senkaku islands, because it will unilaterally escalate the situation and raise tensions in the area," it said.
Saturday's protest came a day after Japanese vice-foreign minister Shinsuke Sugiyama summoned Cheng Yonghua, Beijing's ambassador to Tokyo, to protest over intrusions into its territorial waters by Chinese coast guard and fishing vessels on Friday afternoon.
Tensions over the islands have seriously harmed bilateral relations.
The two sides have gradually taken steps to ease tensions through dialogue but the fundamental divide over the islands remains unresolved and tensions occasionally flare up.
Japan also lodged a protest in June after it said a Chinese navy frigate sailed close to territorial waters near the islands for the first time.