All six Chinese surveillance ships have left Japanese-controlled waters after briefly conducting a patrol mission near a group of disputed islands.

Japan's Coast Guard say the ships left the area surrounding the uninhabited archipelago Friday after both sides exchanged warnings in the contested waters.

Japan had organized an emergency task force and summoned the Chinese ambassador in response to the move, which it called “regrettable” and “unprecedented.”

Chinese foreign ministry spokesperson Hong Lei said the “routine law enforcement” mission was “completely justifiable,” saying the current tensions between Tokyo and Beijing are “completely caused by Japan.”

China's official Xinhua news agency said the mission dealt “a big blow to the inflated swagger of Japan.”

Tensions surrounding the rocky islets, known as Senkaku in Japan and Diaoyu in China, have heightened since Tokyo angered China by purchasing them this week from a private Japanese landowner.

Since then, anti-Japan protests have broken out across China. On Friday, dozens of protesters waved Chinese flags and chanted slogans as they marched outside Japan's embassy in Beijing, calling for Japan to leave the uninhabited islands.

The protests came as the Japanese consulate in Shanghai reported that a group of Japanese citizens were hurt during an assault in a Chinese restaurant. Their injuries were minor. Japan's foreign ministry has warned its citizens in China to be aware of anti-Japan protests and not draw attention to themselves.

China's vice commerce minister, Jiang Zengwei, warned Thursday the dispute could affect trade between China and Japan, while Japanese Foreign Minister Koichiro Gemba called for calm.

China is Japan's largest trading partner.

On Monday, Japan announced a $26 million deal to nationalize the disputed island chain, whose waters contain rich fishing grounds and potential oil reserves. Japanese officials said the move was meant to ensure that no one triggers a confrontation with China by developing the uninhabited islands.

China called Japan's purchase a violation of Chinese sovereignty, saying China does not recognize any Japanese ownership of the islands. China urged Japan to revoke the purchase immediately.

Japan rejected China's demand, saying Tokyo will not reconsider a transaction involving what it considers to be sovereign Japanese territory.