Two Japanese activists landed on an island at the centre of a bitter dispute with China on Tuesday, the government in Tokyo said, as fresh anti-Japanese protests rocked Chinese cities.

The landing came as a Chinese fisheries patrol boat sailed to waters near the islands, although it did not enter Japanese territory.

"Two Japanese landed on Uotsurijima at about 9:30 am (0030 GMT)," Chief Cabinet Secretary Osamu Fujimura told reporters, referring to the largest island in the chain. "The coastguard said they have already left," he said.

Jiji Press, citing police in Okinawa, said the two were from Japan's main southern island of Kyushu.

They arrived in the area in a small boat and swam to the island, a spokesman at the coastguard in Okinawa said, adding they were back in their boat shortly afterwards.

The landing, the fourth by Japanese this year, came after coastguards warned away a Chinese fisheries patrol boat spotted near the island.

The ship was sighted 42 kilometres (26 miles) north-northwest of Uotsurijima at 7:00am (2200 GMT Monday), left "contiguous" waters five hours later, and then re-entered the area one hour later, the coastguard said.

"Our patrol vessels are warning it not to enter our country's territorial waters by radio and other means," the coastguard said in a statement.

The ship has earlier told Japanese ships that it was "carrying out legitimate activity", arguing the islands are Chinese sovereign territory.

Widespread anti-Japanese protests, some of them violent, have been held across China in recent days over the East China Sea islands known as Diaoyu by Beijing and Senkaku by Tokyo. They are claimed by both but controlled by Japan.

Major Japanese firms including Canon and Honda have suspended operations at several plants in China, according to officials and reports Monday.

After meetings in Tokyo with senior Japanese officials on Monday, US Defense Secretary Leon Panetta -- who later travelled on to Beijing -- urged "calm and restraint on all sides".

A fresh wave of anti-Japan rallies was in progress Tuesday, the anniversary of the 1931 "Mukden incident" that led to Japan's invasion of Manchuria, which is commemorated every year in China.

China and Japan have close economic and business ties, with two-way trade totalling $342.9 billion last year, according to Chinese figures.

But the two countries' political relationship is often tense due to the territorial dispute and Chinese resentment over past conflicts and atrocities.

A landing on the island by pro-Beijing nationalists in August marked the start of a sudden worsening of relations between China and Japan.

Tokyo announced last week it had bought three of the islands, which it administers, from their private Japanese landowner.

A Taiwanese politician said Tuesday a group of Taiwanese fishermen were planning to sail this week to the archipelago.

About 60 fishing boats each carrying five to six people are expected to head for the islands Saturday from a port in northeast Taiwan's Ilan county, said Lin Chi-shan, a co-organiser of the event and a member of Ilan county council.

Taiwan also claims the islands, which are uninhabited but strategically important.