At least 10 Japanese nationalist activists have landed on a group of disputed islands, amid an escalating territorial row with China.
The activists swam ashore after a flotilla carrying about 150 people reached the Japanese-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 China and strong condemnation from Beijing.
More than 100 people gathered near the Japanese consulate in China's southern city of Guanghzou, demanding the Japanese to 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 they landed on the rocks, emulating the pro-China activists who had made the same gesture during their trip.
A journalist from the French AFP news agency, who is aboard one of the boats, said the flotilla had arrived off the main island, Uotsuri, at sunrise.
One of the politicians on the flotilla, Kenichi Kojima, told AFP: "I want to show the international community that these islands are ours. It is Japan's future at stake."
China has warned that the expedition would undermine its territorial sovereignty.
"Any unilateral action taken by Japan on the Diaoyu Islands is illegal and invalid," said Chinese foreign ministry spokesman Qin Gan.
Earlier this week, pro-Chinese activists sailed to the disputed island chain from Hong Kong in a protest aimed at promoting Chinese sovereignty.
Some of the activists were deported by Japan, and others sailed away from the islands.
In a separate development, Japanese newspaper Yomiuri Shimbun is reporting that Tokyo is planning to replace its ambassador to China.
Ambassador Uichiro Niwa earlier this year warned that a plan put forward by Tokyo's municipal government to buy some of the islands could spark a "grave crisis" between Japan and China.
The government was reportedly furious with him for misrepresenting Tokyo's position on the islands.
Rows over the disputed islands have caused Sino-Japanese ties to freeze in the past.
In September 2010, relations plummeted after the arrest of a Chinese trawler captain near the islands.
The captain was accused of ramming two Japanese patrol vessels in the area, but Japan eventually dropped the charges against him.
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.