At least 10 Japanese nationalist activists have landed on a group of disputed islands, amid an escalating territorial row with China.
The BBC reports the activists swam ashore after a flotilla carrying about 150 people reached Japan's controlled Senkaku islands, called Diaoyu in China.
Japan's coast guard is now questioning the activists, who had earlier been denied permission to visit the islands.
The move triggered protests in a number of cities across China.
More than 100 people gathered near the Japanese consulate in China's southern city of Guanghzou, demanding the Japanese leave the islands, the state-run Xinhua news agency said.
Anti-Japanese rallies were also held in Shenzhen, Qingdao and Harbin, the agency reported.
The islands in the East China Sea lie on a vital shipping lane, and are surrounded by deposits of gas.
They are also claimed by Taiwan.
The Japanese launched their flotilla on Saturday, saying they wanted to commemorate the Japanese who died near the islands in World War II.
They raised the Japanese flag when the landed on the rocks, emulating the pro-China activists who had made the same gesture during their trip.
China has warned that the expedition would undermine its territorial sovereignty.
China claims the islands have been a part of its territory since ancient times, but Japan says it took control of the archipelago in the late 1890s after making sure they were uninhabited.