BEIJING - Two Chinese marine surveillance ships reached waters near the disputed Diaoyu islands, as tensions rose after Japan ignored Chinese protests and formally bought the isles it calls Senkaku from a private Japanese owner.
Beijing, meanwhile, announced the geographical coordinates of its territorial waters around the group of five isles and three barren rocks in the East China Sea, as a possible legal basis for its claim.
To assert its claim to ownership, the Chinese say they will have "normalised" surveillance of the area and even provide daily weather reports.
These actions, which analysts say are Beijing's strongest yet, come as Japan signed a 2.05 billion yen (S$32.4 million) deal to buy three of the isles.
But Beijing says they are not Japan's to buy. "We seriously declare that Japan's so-called purchase is totally illegal and ineffective," said Chinese foreign ministry spokesman Hong Lei.
China would take "necessary measures" depending on the situation to assert its sovereignty, but gave no details, he said.
China's top legislative and consultative bodies also urged Japan to rethink its move. China is no longer a poor and weak country that can be carved up at will, they said, adding that Japan should desist from "playing with fire".
The official China Daily even said: "A thief is never a legitimate owner of stolen property."
Japan's action sparked public protests from Shandong to Guangdong. In Beijing, about 40 men and women unfurled a Chinese flag in front of the Japanese embassy and shouted, "Little Japan, get out of Diaoyu islands!"
Japanese Prime Minister Yoshihiko Noda justified the purchase, saying this would put the contested turf under state control and rein in provocations, an oblique reference to right-wing Tokyo governor Shintaro Ishihara's bid to buy and develop them.
Analyst Li Mingjiang said this was a valid point: "If the islets fell into the hands of Ishihara's local government, it would be a nightmare for Japan-China ties."
Japan owns one of the five Senkaku islets. It bought three of the four privately owned ones yesterday. The fourth has been leased by the Defence Ministry for use by the American military.
Japan yesterday sent a senior foreign ministry envoy to Beijing to brief Chinese officials.
Foreign Minister Koichiro Gemba said: "The latest development should not ever disturb the stable progress of bilateral ties between Japan and China."
But Chinese analyst Wang Xiangsui said the dispute had sailed into uncharted waters, escalating from public and diplomatic protests to state-level actions.
He said this was likely to herald the start of antagonism between China and Japan over the islands for the mid- to long-term.
