Japan says China has sailed a fleet of 230 vessels near Japanese-controlled waters in the East China Sea.
The fleet included fishing boats and coastguard ships, Japan says, and three vessels appeared to be armed. Officials have protested to Chinese diplomats.
The reported incident occurred near the Japan-controlled disputed islands, known as the Senkaku islands in Japan and the Diaoyu islands in China.
Beijing has been increasingly assertive about waters it believes are Chinese.
In a separate incident on Saturday, Chinese state media said fighter jets and bombers had completed a patrol of airspace above islands in the South China Sea, as part of combat training.
These islands are also disputed, but last month an international tribunal dismissed most of Beijing's claims in that sea.
China said it would ignore the decision.
Testing Japan's resolve
A Japanese foreign ministry spokesman said the fleet's despatch to the East China Sea islands was a unilateral escalation of tension in the area.
Less than a decade ago, Tokyo and Beijing talked of jointly exploiting the resources of the East China Sea, the waterway that separates the two countries.
But since then tension has increased, particularly over the disputed Senkaku/Diaoyu islands, which are uninhabited.
Over recent years, China has sent an increasing number of ships towards the islands, in what appears to be an attempt to test Japan's resolve to defend them.