A group of islands is the source of rising tensions between Japan and China.
Japanese police have questioned 10 people who landed on one of the disputed islands on Sunday, on suspicion of entering land controlled by the government without permission.
This landing comes after another group of activists from a private Hong Kong group illegally landed on one of the islands last week.
Protests have broken out in more than 10 Chinese cities, including Chengdu and Sichuan province. Protestors marched on major roads and waved Chinese flags.
The uninhabited outcrops were the scene of a spat in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of its coast guard vessels.
The islands are administered by Japan, but claimed by both China and Taiwan.