Six Chinese ships have sailed into waters around a disputed archipelago, with Beijing saying they were there for "law enforcement" around islands Japan nationalised earlier this week.
The move, dubbed "unprecedented" by Tokyo, came on Friday as it was reported Japanese nationals had been physically attacked in China, marking the latest stage in a deteriorating row between Asia's two biggest economies.
Japanese living or visiting China were warned to take extra precautions after assaults and harassment were reported to the consulate in Shanghai, a base for Japanese businesses and a popular tourist destination.
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.
Al Jazeera's Marga Ortigas, reporting from Hong Kong, said protesters in Beijing have called for a boycott of Japanese products to defend their country’s interests.
The latest tension surfaced after Yoshihiko Noda, the Japanese prime minister, announced that Japan is buying the disputed private islands and transferring control to the government.
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.
"We'll do our utmost in vigilance and surveillance," said Noda when asked about Japan's response to the latest move by China. He also established a task force to deal with the issue.