Tokyo - Six Chinese ships sailed into waters around a disputed archipelago on Friday, with Beijing saying they were there for "law enforcement" around islands Japan nationalised earlier this week.
The move - dubbed "unprecedented" by Tokyo - marked the latest stage in a deteriorating row between Asia's two biggest economies and came as reports emerged of Japanese nationals being physically attacked in China.
Tokyo summoned the Chinese ambassador to protest what it insists is an incursion into territorial waters around islands it controls, called Senkaku, but claimed by Beijing as Diaoyu.
However, China was resolute, with the foreign ministry issuing a forthright statement claiming the boats were patrolling sovereign territory.
"Two Chinese surveillance ship fleets have arrived at waters around the Diaoyu Islands and adjacent islands on 14 September 2012 to start patrol and law enforcement," the statement said.
"These law enforcement and patrol activities are designed to demonstrate China's jurisdiction over the islands and safeguard its maritime interests."
Coastguard on scene
Japan's coast guard said the ships had all arrived by 07:00, with three of them having left the area by 08:30.
Coastguard vessels were on scene, demanding the ships leave, a spokesperson said.
Japanese Prime Minister Yoshihiko Noda established a task force to deal with the issue on Friday morning and his foreign ministry summoned China's ambassador, Cheng Yonghua, to lodge a protest.
"We understand that [the dispatch of] six ships is surely an unprecedented case, considering past incidents," Chief Cabinet Secretary Osamu Fujimura told reporters.
Fujimura said Yonghua had reiterated Beijing's claims to the islands in the East China Sea, which lie around 400km from the Okinawan capital of Naha and 200km from Taiwan.
"The Japanese government stated its position firmly about the issue of Senkaku, while asking him about the safety of Japanese nationals in China."
Japan urged "that China do its utmost - above all else - to secure the safety of Japanese nationals in China", he said.
Fujimura's comments come as the Japanese consulate in Shanghai reported physical attacks on its nationals.
"A group was dining late at night, and they were harassed and assaulted by Chinese," said a statement on the consulate's website.
The consulate said bottles were thrown at some Japanese, and drinks and food were poured over others, while one person reported having a pair of glasses broken.
Japan's foreign ministry has warned its nationals who are in China or who are planning to visit there to be aware of anti-Japanese demonstrations and to avoid drawing attention to themselves.
The embassy in Beijing said Japanese should avoid approaching the building, where protests have been reported, unless absolutely necessary.
Relations between the two countries - often rocky because of a difficult history - have worsened since pro-Beijing nationalists landed on one of the islands in August.
They were arrested and deported by Japan, but followed days later by Japanese nationalists, who raised their flag there.
Anti-Japanese protests broke out in China and have continued since Japan on Tuesday announced it had nationalised three of the islands in the chain. It already owns another and leases the fifth.
The purchase was intended at least partially to head off an attempt to buy them by Tokyo's provocative Governor Shintaro Ishihara, who charged Japan was not doing enough to protect its territory.
Commentators say Noda's solution - nationalising the islands and continuing its policy of doing nothing with them - was an attempt to navigate between rising nationalism at home and China's growing assertiveness on the oceans.
But Beijing's reaction has been sharper than many analysts expected. Some observers have pointed to the forthcoming leadership change in China's Communist Party and say the islands issue is being used as a way to distract public attention from the less-than-smooth transition.