China has carried out the first round of countermeasures against Japan after its government signed a contract to buy the Diaoyu Islands on Tuesday to "nationalize" the islands, by sending two law enforcement vessels to the waters near the islands.

Following strong opposition from China's top leaders and the government on Monday, the top legislature, military and public voiced protests against Japan's unilateral move amid soured ties between the two sides.

Two ships of the China Marine Surveillance (CMS), the 1,100-ton-class Haijian-46 and 996-ton-class Haijian-49, reached the waters around the Diaoyu Islands Tuesday morning to demonstrate China's undisputable sovereignty over the islets, the Xinhua News Agency reported.

Meanwhile, the State Oceanic Administration Tuesday formally introduced maritime environment forecasting for the sea areas around the islets.

Xinhua quoted sources from the CMS as saying that it has drafted an action plan for safeguarding sovereignty and would take actions pending the development of the situation.

When contacted by the Global Times, the CMS declined to reveal the patrol route and specific patrol area of the vessels. Meanwhile, the Japanese Coast Guard (JCG) told the Global Times that it couldn't confirm whether the vessels had entered waters near the islets.

Jin Yongming, a researcher on law of the sea with the Shanghai Academy of Social Sciences, told the Global Times that the patrol mission and environment forecasting are follow-ups to the government's announcement of the base points and baselines of the territorial waters of the Diaoyu Islands on Monday.

According to Jin, the effectiveness of the patrol mission depends on whether the two vessels enter waters within 12 nautical miles of the Diaoyu Islands. "It is inevitable for the surveillance ships to clash with the JCG if they enter the area," said Jin, noting that the Chinese navy should be prepared for the scenario and play its due role.

The Ministry of National Defense Tuesday posted a statement on its official website protesting Japan's purchase of the islets. Geng Yansheng, spokesman of the ministry, said in the statement that the Chinese government and army are unshakeable in their determination and will safeguard the country's territorial sovereignty. "We are closely following the development of the issue, and reserve the right to take reciprocal measures," said Geng.

In recent days, three Chinese military areas, Guangzhou, Chengdu and Jinan, staged a series of military drills, which led to speculation that they were targeting the dispute surrounding the Diaoyu Islands.

According to China Central Television, troops from the Jinan military area last week held an exercise, simulating the seizing of an inhabited islet.

Japanese Prime Minister Yoshihiko Noda Tuesday ordered Japan's Self Defense Forces to be fully prepared for any emergency under the complex peripheral security environment, reported Xinhua. Noda particularly emphasized that China has been increasingly active in the surrounding seas.

The Japanese government does not accept the protest from Chinese authorities over the purchase and relevant works will be implemented as scheduled, Kyodo News reported.

Jin said the strong opposition from top Chinese leaders over Japan's move and legal steps taken by the government indicate that Beijing will not back down in the diplomatic showdown. "It's a pressing matter to formulate an emergency response mechanism headed by top-level officials to coordinate countermeasures in the sea," he said.

According to Lü, the disputes would definitely hurt Sino-Japanese exchanges at all levels, noting that the unilateral move by Japan violated the preconditions for high-level consultations between Tokyo and Beijing, and that Japan should take the blame for cooling ties.

"Though the two sides are highly interdependent in economy and trade, it's inevitable that economic cooperation between the two sides will be affected," said Lü.

Japan's signing of the island purchase contract with the so-called private owner on Tuesday also triggered small-scale protests in China.

More than 10 protesters gathered at the Japanese embassy in Beijing on Tuesday, holding placards that read, "The Diaoyu Islands are Chinese territory," reported Xinhua.

About 200 people in Weihai, Shandong Province, marched on the street for an hour, chanting slogans about safeguarding the islets. Some Web users even suggested boycotting Japanese goods, said Xinhua.

About 20 percent of tourists bound for Japan during the forthcoming weeklong Mid-Autumn Day and National Day Holiday have canceled their travel plans due to safety concerns after the incident, according to the China International Travel Service. Similar actions also occured in Beijing, it said.

According to Reuters, Japan said it is sending its Asia department chief to Beijing on Tuesday for talks to "avoid misunderstanding and lack of explanation on the issue."

Meanwhile, US State Department spokeswoman Victoria Nuland told a daily press briefing that Washington wants to see the dispute between Beijing and Tokyo handled calmly and through dialogue.

Lin Meilian and agencies contributed to this story

Related report:

Discard illusion of friendly ties with Japan