Thousands of anti-Japan protesters rallied across China over a territorial row on Tuesday, a key historical anniversary, as Japanese firms including car giant Toyota shut or scaled back production.

The demonstrations came after several days of protests, some of them violent, over disputed islands in the East China Sea that have raised international concerns and fears of conflict between two of the world's top three economies.

US Defense Secretary Leon Panetta, who is currently on a trip that takes in both Asian giants, was to call for calm in meetings with Chinese officials, after warning of the risk of a "misjudgment" that "could result in violence".

The islets, known as Senkaku in Japan and Diaoyu in China, are uninhabited but situated in rich fishing waters and said to sit atop valuable natural resources. They are controlled by Japan but claimed by China and Taiwan.

Toyota, the world's biggest automaker, declined to offer specifics on shutdowns at its three assembly plants and six other factories in China, saying only that it viewed "employees' safety to be top priority".

"Some (factories) will operate and some will not," a spokesman said.

Honda Motor, which makes about 970,000 vehicles a year in China, said it had closed all five of its plants in the country for Tuesday and Wednesday, while Nissan -- which has China as its biggest market -- temporarily shut two of three factories.

Electronics giants Canon and Panasonic have also said they were temporarily shutting some China operations, and a group of Japanese business leaders headed by Toyota's chairman said it may cancel a trip to China because of the dispute.

China and Japan have close trade and business ties, with numerous Japanese companies investing in their larger neighbour and 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.

Tuesday marked the September 18, 1931 "Mukden Incident" in which Japanese soldiers blew up a railway in Manchuria as a pretext to take control of China's entire northeastern region, which is commemorated every year in China.

Chinese state television showed sirens being sounded at 9:18 am -- symbolising the date -- as a reminder to "remember the history and not forget national disgrace", it said.

Armed police were deployed in force at protests across the country, with some marchers carrying portraits of longtime leader Mao Zedong and urging Beijing to stand up to its historic rival.

Outside the Japanese embassy in Beijing more than 1,000 protesters faced off against riot police six rows deep.

Some threw eggs and plastic bottles at the building and a few scuffles broke out with officers at the gate of the compound.

"Today is September 18, the anniversary of when Japan invaded China's northern area. It is a good time to show them that we are prepared to fight," said Fan Li, 31, wearing a T-shirt reading: "Diaoyu Islands are Chinese."

Many protesters carried banners calling for boycotts of Japanese goods, and others sang the national anthem.

In the commercial hub of Shanghai hundreds of riot police blocked off roads leading to the Japanese consulate, while around 3,000 of protesters rallied outside the building, carrying flags, banners and images of Mao.

"I worship Mao. If we still had Mao, then we would just go fight Japan," said Pu Lingkuang, 34, holding up a large portrait.

Minor scuffles broke out in Shenzhen, an AFP photographer saw, and hundreds of protesters marched in Chengdu.

The row over the islands intensified last week when the Tokyo government bought three of them from their private Japanese owners, effectively nationalising them. China responded by sending patrol ships to nearby waters.

In August, pro-Beijing nationalists landed on one of the islands, setting off the current tensions.

Two Japanese activists landed on one of the islands Tuesday but later left, Tokyo said, while coastguards warned away a Chinese fisheries patrol boat spotted nearby.