Anti-Japan protests have reignited across China as a crisis over a territorial dispute escalated on the anniversary of Japan's pre-war invasion of its giant neighbour.
The protests forced Japanese firms in the country to suspend operations.
Relations between Asia's two biggest economies have faltered badly on the anniversary, with emotions running high on the streets and out at sea where two Japanese activists landed on an island at the centre of the dispute.
China reacted swiftly to the news of the landing, which risked inflaming a situation that already ranks as China's worst outbreak of anti-Japan sentiment in decades.
Beijing described the landing as provocative, lodged a complaint with Tokyo and said it reserved the right to "take further action".
"The unlawful landing of the Japanese right-wingers on the Chinese territory of the Diaoyu islands was a gravely provocative action violating Chinese territorial sovereignty," Chinese Foreign Ministry spokesman Hong Lei said in a statement.
The dispute over the uninhabited group of islands in the East China Sea - known as the Senkaku in Japan and Diaoyu in China - led to a day of anti-Japan protests which Japanese expatriates fear could peak later today.
Japanese businesses shut hundreds of stores and plants across China and Japan's embassy in Beijing again came under siege by protesters hurling water bottles, waving Chinese flags, and chanting anti-Japan slogans evoking war-time enmity.
"Wipe out all Japanese dogs," read one banner held aloft by one of thousands of protesters marching on the embassy, which was ringed by riot police standing six rows deep. Japan's foreign ministry said some embassy windows had been smashed.
Rowdy protests, fuelled by Chinese nationalism, sprang up in other major cities including Shanghai, raising the risk they could get out of hand and backfire on Beijing, which has implied tacit approval to them through state media.
One Hong Kong newspaper said some protesters in southern Shenzhen had been detained for calling for democracy and human rights.
US Defense Secretary Leon Panetta, visiting China to promote stronger Sino-US military ties, again called for calm and restraint.
Washington has said it will not take sides in the dispute, although it is a strong ally of Japan.
Well-known Japanese firms have been targeted by protesters, with car makers Toyota Motor Corp and Honda Motor Co halting some operations after attacks on their outlets.
Other Japanese companies also shuttered plants and stores in China, sending Japanese share prices falling and prompting a warning from credit rating agency Fitch that the situation could hurt some auto and tech firms' creditworthiness.
Hitachi Construction Machinery recalled 25 Japanese workers back to Japan because of the unrest.
Japanese restaurants, a common target of protesters, barred their doors while many Japanese expatriates stayed home, afraid that Tuesday's anniversary of Japan's 1931 occupation of parts of mainland China could lead to outbreaks of violence.
The brief landing by two Japanese nationals on one of the disputed islands, reported by Japan's coast guard, has raised fears of a direct clash in an area being patrolled by ships from both nations.
The activists briefly landed on one of the islands, having paddled up to it in a rubber raft and swam ashore before returning to the boat, Japanese broadcaster NHK said.
A flotilla of around 1,000 Chinese fishing boats is also reported by Chinese and Japanese media to be heading to the area, which contains potentially large gas reserves.
In 2010, a bilateral crisis over the islands erupted after a fishing boat collided with a Japanese Coast Guard vessel.
The Japanese government has set up an information-gathering operation to monitor the movements of the Chinese fishing boats.
