Tens of thousands of anti-Japan demonstrators rallied across China over disputed islands on Tuesday, a key historical anniversary, as Tokyo said 11 Chinese government vessels had entered the area.
The moves followed several days of protests, some of them violent, that have raised international concerns and fears of conflict between two of the world's top three economies, with Japanese firms shutting or scaling back production.
Several thousand people gathered outside the Japanese embassy in Beijing, some throwing eggs and plastic bottles and others carrying portraits of Mao Zedong, the divisive late leader venerated by some for standing up to Japan.
"China is not a weak country any more. We are strong and we should no longer be bullied by Japan," said Jiu Longtou, a 31-year factory worker. "Diaoyu Island is Chinese and we should protect it from Japan."
A Japanese coastguard spokesman said 10 Chinese maritime surveillance ships and a fisheries patrol boat had entered an area known as the contiguous zone around the East China Sea islets, with three going into territorial waters.
No immediate comment was available from Chinese ministries and authorities.
But China's defence minister Liang Guanglie said earlier Beijing reserved the right to take "further actions" over the islands -- which are controlled by Tokyo -- while hoping for "a peaceful and negotiated solution".
At a joint press conference, his US counterpart Leon Panetta urged "calm and restraint by all sides".
A financial spat between Asia's two biggest economies could cast a pall over growth on the continent, which major Western countries are counting on to drive recovery from the global slowdown.
The islets, known as Senkaku in Japan and Diaoyu in China, are uninhabited but situated in rich fishing waters and are said to sit atop valuable natural resources.
They are controlled by Japan but claimed by China and Taiwan, and the current row has roiled the political relationship between Beijing and Tokyo, which is coloured by Chinese resentment over past conflicts and atrocities.
Tuesday marked the anniversary of 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.
Tokyo was pressing China through diplomatic channels to protect Japanese citizens and firms in China, said the Japanese government's top spokesman.
Despite the tensions, 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.
Toyota, the world's biggest automaker, declined to offer specifics on shutdowns at its three assembly plants and six other factories in China. "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 temporarily shut two of its three factories.
Electronics giants Canon, Panasonic and Sony also said they were temporarily shutting some China operations, while the Uniqlo clothing chain's owner Fast Retailing shut dozens of outlets, as did 7-Eleven convenience store operator Seven & i Holdings.
Armed police were deployed in force at protests across China. Some marchers carried banners that denounced Japan or called for boycotts of Japanese goods, while others urged Beijing to stand up to its historic rival.
Protesters in the capital carried signs reading "Kill Japanese", placards with the face of the Japanese prime minister on the body of a dog, and a picture showing a Chinese soldier stabbing a Japanese enemy with a sword.
In the commercial hub of Shanghai, where 3,000 protesters rallied outside the Japanese consulate, some carried banners reading: "Leave Japanese dogs" and "Exterminate the Japanese".
"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 the southern city of Shenzhen and hundreds of protesters marched in Chengdu, in the southwest.
In August, pro-Beijing nationalists landed on one of the islands, setting off the current tensions.
The row intensified last week when the Tokyo government bought three of the islands from their private Japanese owners, effectively nationalising them.