Demonstrations have been held in China and Japan about islands claimed by both countries in the East China Sea.
In Tokyo, demonstrators rallied against China's claim to the islands and delivered a note to China's embassy.
In China, protesters chanted anti-Japanese slogans in three cities.
The long-running row erupted into a full-blown diplomatic crisis last month when Japan seized a Chinese fishing boat and its captain after a collision in disputed waters.
The captain was accused of deliberately ramming two Japanese patrol boats. He was released after two weeks, and the two countries have since moved to ease tensions.
However China has demanded an apology over the incident - something Japan has rejected.
During Saturday's demonstration in Tokyo, hundreds of people carried banners reading "Japan is in danger" and "Don't forgive invader China".
Beijing authorities later expressed "deep concern" over the protest.
In China, anti-Japanese demonstrations were reported in the cities of Chengdu, Xian and Zhengzhou.
Several thousand protesters carried banners with slogans such as "Overthrow Japanese imperialism" and "Protect the Diaoyu islands."
Witnesses said at least one Japanese restaurant had its windows broken.
There have been demonstrations over the uninhabited islands, which are known as Senkaku in Japan and may have rich oil and gas deposits, on several occasions in China in the past.