Two Chinese patrol ships have been sent to islands disputed with Japan, which has sealed a deal to purchase the territory, Chinese state media say.
The ships had reached waters near the islands - known as Senkaku in Japan and Diaoyu in China - to "assert the country's sovereignty", Xinhua said.
Japan confirmed on Tuesday it had signed a contract to buy three of the islands from their private owner.
Tension has been rumbling between the two countries over the East China Sea.
Japan controls the uninhabited but resource-rich islands, which are also claimed by Taiwan. Some had been in the hands of a private Japanese owner but the government says it has now signed a purchase contract.
"This should cause no problem for Japan's ties with other countries and regions," said Chief Cabinet Secretary Osamu Fujimura.
"We have absolutely no desire for any repercussions as far as Japan-China relations are concerned. It is important that we avoid misunderstanding and unforeseen problems."
Mr Fujimura told reporters that the government had set aside 2.05bn yen ($26m, £16.4m) to pay for the three islands.
Japan said on Monday that it was buying the islands to promote their stable and peaceful management.
But the move followed a bid by the outspoken and right-wing Tokyo Governor Shintaro Ishihara to buy them using public donations - an action analysts believe would have further raised tensions with China.
China has nonetheless called Japan's move illegal and warned it would affect ties.
"The Japanese plan to purchase the islands is completely illegal and invalid... It cannot change the reality that Japan is seeking to steal the islands," Foreign Ministry spokesman Hong Lei said during a regular press briefing on Tuesday.
"The well-being of the development of the Sino-Japan relationship needs mutual understanding. The move to purchase the Diaoyu islands betrays these principles."
State-run media have also carried strongly worded statements on the issue.
"The Chinese government will not sit idly by watching its territorial sovereignty being infringed upon," the Foreign Ministry said in a statement issued on Monday and carried by Xinhua news agency.
"Should the Japanese side insist on going its own way, it shall have to bear all serious consequences arising therefrom."
Chinese Premier Wen Jiabao also reiterated China's stand on Monday.
"The Diaoyu islands are an inalienable part of China's territory, and the Chinese government and its people will absolutely make no concession on issues concerning its sovereignty and territorial integrity," he said.
The announcement of the dispatch of the patrol boats came in a brief Xinhua report.
China Marine Surveillance - a maritime law enforcement agency - had "drafted an action plan for safeguarding the sovereignty and would take actions pending the development of the situation", it said.
A small group of protesters gathered at the Japanese embassy in Beijing to protest against the purchase.
Taiwan has also lodged a formal protest over the issue, calling it an "extremely unfriendly move".
The islands, which lie south of Okinawa and north of Taiwan, sit in key shipping lanes and are thought to lie close to gas deposits.
Analysis Jonathan Marcus BBC Diplomatic Correspondent
This is a row as much about politics and emotion as the strategic significance of the islands themselves. Domestic uncertainty in both countries plays a part too.
In China there are tensions at the highest levels in Beijing ahead of impending leadership changes and in Japan, the weakness of the current government has left it open to manipulation by more strident nationalist voices.
Indeed nationalism and history play a central part in this dispute. The resurgence of nationalist strains in both countries serves to open up old wounds stemming from the early 1930s when Japanese forces rampaged through Manchuria.
The dispute is as much about self-perception as anything else: China the rising power in the region and Japan, perhaps less secure, with its economic miracle in the past, fast re-aligning its defence policy to cope with what it sees as Beijing's growing assertiveness.
Japan-China disputed islands
The archipelago consists of five islands and three reefs
Japan, China and Taiwan claim them; they are controlled by Japan and form part of Okinawa prefecture
The Japanese government signed a deal in September 2012 to purchase three islands from Japanese businessman Kunioki Kurihara, who used to rent them out to the Japanese state
The islands were the focus of a major diplomatic row between Japan and China in 2010
First of all, every reader in Europe who wishes to know what happend here in East Asia must read the following history:
1) The Diaoyu islands were part of the territory called Ryukyu Islands (kingdom), which were occupied by aboriginals like Guan. The Ryukyu king sent officers to Ming dynasty (China) to serve as affiliated kingdom to Ming emperor in 1500s - 1600s. After Qing(sino)-Japan war in 1800s, Japan grabbed the Ryukyu kingdom (including Okinawa) and Taiwan from Qing, and separated the Ryukyu by upside and downside parts. The down part, especially the Diaoyu islands, was sent to Japanese occupied Taiwan government and documented. A Japan fisher rented the barren islands then.
2) After WWII, US planned to send the Ryukyu islands including Diaoyu islands to Chiang-kai-shek who fled from mainland to Taiwan and formed up a pro-US country which is nowadays ROC/Taiwan. Chiang-kai-shek refused because he didn't have enough military force to do so. In 1951, Japan signed "Treaty of Peace with Japan" with allies admitting that the Ryukyu islands are under trusteeship by United Nation. Everything was kept quiet during Korean war, because all three parties: Mainland China, Taiwan and Japan were all too weak to take care of the no-man islands. The USCAR, United States Civil Administration of the Ryukyu Islands, governed the whole territory during that time. In 1972, US began to retreat from the islands, and left the islands ungoverned. Because US did not think about which party, Japan or Taiwan, would govern the islands, it simply leaves the islands to Japan.
3) In 1970s, when the oil was discovered, three parties began to argue the belonging of the oil field and the belonging of the Diaoyu islands. In fact the first parties who declared protests against Japan in 1970s was not China, but Taiwan and Hong Kong (governed by United Kingdom). Protests reached to top in later 1970s when thousands of sampans were sent to Diaoyu islands by Taiwan patriots (including nowadays Taiwan president Mr. Ma Ying-Jeou)
4) From the three points, it is very clear that the islands should be governed by Taiwan.
5) It was because Taiwan now is so weak, so that China is playing protester against Japan. If Taiwan could play a tough role, then I believe everything could be solved easily, because Taiwan and Japan are within the Asia framework created by US. But it is without doubt that Diaoyu islands are Taiwan territory. It would be a further talk between Taiwan and Mainland China on how to separate the fishing rights and oil digging in the future. Japan should not take part in any of these disputes.
Nobody wants a war in East Asia. But Japanese government is going too far now for its political general election show.