Beijing dispatched two ships to the islands, which Tokyo says it has purchased from its self-proclaimed, Japanese owner.
The two marine surveillance ships had reached the waters around the Diaoyu islands – known in Japan as the Senkaku islands – and would "take actions pending the development of the situation," China's state Xinhua news agency said.
The arrival came as the Japanese government announced it had completed its planned purchase of the islands, which lie in a strategically important shipping area with valuable mineral resources thought to be nearby.
"This should cause no problem for Japan's ties with other countries and regions," said Japan's 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," he told reporters.
Beijing had earlier summoned the Japanese ambassador and lodged a strong protest over Tokyo's move to purchase the islands, while vowing to take countermeasures.
'Never ever yield'
Chinese Premier Wen Jiabao said the islands were "an inherent part of China's territory" and vowed his country would "never ever yield an inch" on its sovereignty.
"We have been monitoring the developments of the situation closely and will take necessary measures to uphold China's territorial sovereignty," China's foreign ministry spokesperson Hong Lei told reporters.
Often testy Japan-China ties took a turn for the worse in August when pro-Beijing activists landed on one of the islands.
They were arrested by Japanese authorities and deported. Days later about a dozen Japanese nationalists raised their country's flag on the same island, Uotsurijima, prompting protests in cities across China.
Japan's government currently leases four islands and owns a fifth. It does not allow people to visit and has a policy of not building anything there.
State television and all major daily newspapers in China on Tuesday highlighted Beijing's condemnation of the purchase, in what appeared to be an official effort to stoke patriotic sentiment over the issue.
The islands, which lie around 200km from Taiwan and 2 000km from Tokyo, are also claimed by Taipei, which strongly protested the Japanese move on Tuesday.
"We strongly demand that the Japanese government revokes this move," Taiwan's foreign minister Timothy Yang told reporters in Taipei.
"Japan's unilateral and illegal action cannot change the fact that the Republic of China [Taiwan's official name] owns the Diaoyu islands."