Six Chinese ships sailed into waters around a disputed archipelago on Friday, with Beijing saying they were there for "law enforcement" around islands Japan nationalised earlier this week.
Japan's Coast Guard told the state surveillance ships to leave the area around the chain, which Tokyo knows as Senkaku and China claims under the name Diaoyu.
The arrival came just days after the Japanese government nationalised the islands, sparking a furious reaction from Beijing in a row that has dominated relations and news coverage for weeks.
"Our patrol vessels are currently telling them to leave our country's territorial waters," the coastguard said in a statement.
However, Beijing stood firm, with the Chinese foreign ministry saying the ships were on patrol.
"Two Chinese surveillance ship fleets have arrived at waters around the Diaoyu Islands and adjacent islands on September 14, 2012 to start patrol and law enforcement," said a statement issued in Beijing.
"These law enforcement and patrol activities are designed to demonstrate China's jurisdiction over the islands and safeguard its maritime interests."
Japanese Prime Minister Yoshihiko Noda vowed to maintain vigilance shortly after the coastguard said two Chinese maritime survey ships had entered Japanese waters around 6:18 am (2118 GMT Thursday).
"We will do our utmost to keep watch," Noda told reporters in Tokyo.
The two ships were followed by a group of four others that arrived in the area shortly after 7:00 am, the coastguard said.
The first two ships left the waters around 7:48 am, the coastguard added.
The disputed archipelago in the East China Sea is around 400 kilometres (250 miles) from Naha, the main city of Okinawa, and 200 kilometres from Taiwan.
Under international law, territorial waters extend up to 12 nautical miles from the coast of a landmass.