Chinese government ships are patrolling near contested East China Sea islands in a show of anger after Tokyo moved to assert its control in the area.
China's official Xinhua News Agency said two patrol ships of the China Marine Surveillance had reached the waters off the islands.
The marine agency is a paramilitary force whose ships are often lightly armed, and Xinhua said it had drawn up a plan of action to safeguard China's sovereignty of the islands.
Beijing warned on Monday that Japan would suffer unspecified consequences if Tokyo purchased the islands from private owners, as it has formally done. Japan already controls the islands, known to Japanese as the Senkakus and to Chinese as the Diaoyus, but Beijing sees the purchase as a further affront to its claims.
The rocky islands have been the focus of recurring spats between the two countries, and are also claimed by Taiwan. The dispute has been heating up in recent months, in part because the nationalist governor of Tokyo had proposed buying the islands and developing them.
Japan's central government announced its own deal this week with the Japanese family it recognises as the owner.
Chief cabinet secretary Osamu Fujimura told reporters the government budgeted 2.05 billion yen (£16.2 million) for the purchase "to maintain the Senkakus peacefully and stably".
Public broadcaster NHK said the government and the family had now signed a deal.
The government does not plan to develop the islands and experts in Japan said the move was meant to block the Tokyo governor's plan, which could have raised tensions further.
Beijing, however, responded with fury, with Premier Wen Jiabao among those warning that China would never back off its claims. China's Foreign Ministry said in a statement on Monday: "If Japan insists on going its own way, it will bear all the serious consequences that follow."