In a statement, Chinese Foreign Ministry spokeswoman Hua Chunying called on Japan to refrain from "any action that could lead to a tense or complicated situation." China's increasingly assertive maritime activities in the East and South China seas have raised concerns and tensions in the region. Japan has joined the U.S., the Philippines and others in urging China to abide by international law after a U.N. arbitration panel ruled in favour of the Philippines in its maritime dispute with Beijing in the South China Sea.
China says it doesn't recognise the July 12 ruling by the Permanent Court of Arbitration.
Relations between Japan and China have been strained over wartime history, a sensitive topic for the region in times of war-end anniversaries during the summer.
China's coast guard vessels routinely sail around the islands, usually in pairs or up to four. Until now, only one of the vessels was armed.
Saturday's fleet size and equipment showed "an escalation of the situation that could heighten tensions in the waters," Japan's Foreign Ministry said, without elaborating on what might have caused the increase.
On Friday, Japan also protested after two Chinese coast guard ships entered the Japanese-claimed waters around the islands.