The "illegal purchase" of the Senkaku Islands "is exerting an increasingly prominent bad impact, and the Japanese side should bear relevant responsibility," Foreign Ministry spokesman Hong Lei said at a regular news conference.
Whether tensions will escalate depends on how Japan acts, Hong said.
"China has stated its position to the Japanese side in different forms, and the Chinese people have also expressed their strong indignation" through protests to denounce the nationalization of the islands, he said. The islands are controlled by Japan but claimed by China as the Diaoyu.
Hong's comments underline the Chinese government's tacit approval of the wave of anti-Japanese sentiment, which began Tuesday and rolled across China as scheduled.
Some of the protests Saturday and Sunday included vandalism, looting and arson targeting Japanese factories, stores and restaurants.
Tuesday will mark the 81st anniversary of the 1931 Mukden Incident, also known as the Manchurian Incident, in which the Imperial Japanese Army blew up a Japanese railway in southern Manchuria to give it an excuse for invading northeastern China.