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.