Two Japanese nationals have landed on one of the islands at the center of a heated territorial dispute with China, as Beijing braces for a day of anti-Japan protests.

Japanese officials say the activists ignored police warnings and swam ashore early Tuesday to the largest of the islands, known in Japanese as Senkaku and in Chinese as Diaoyu. Coast guard officials say they later returned to their boat.

Meanwhile, Chinese state media report that a flotilla of 1,000 Chinese fishing boats is headed to the islands, raising fears of a maritime conflict. Japanese Coast Guard officials have not confirmed the arrival of the boats, but say a Chinese surveillance ship that was cruising near the islands has now left the area.

In Beijing, at least 1,000 protesters marched outside the Japanese embassy, chanting nationalistic slogans and calling for China to assert its claim over the rocky islets. More protests are being held across China on Tuesday, the 81st anniversary of Japan's invasion of northern China.

Tension between Japan and China have heightened since Japan said last week it would purchase some of the islands from their private Japanese landowner. Since then, anti-Japan protests have been held across China. In some cases, the protests have turned violent and have included attacks against Japanese-owned businesses in China.

Several Japanese automakers said Tuesday they were forced to halt operations because of the unrest. Honda says it has closed all five of its Chinese plants, while Nissan has temporarily shut down two of its Chinese factories. Toyota has also scaled back production in China.

Japan and China, Asia's two largest economies, boast strong trade ties. Two-way trade was more than $342 billion last year. But their political relationship is often tense because of the territorial dispute and Chinese resentment about past conflicts.