Japan summoned the Chinese ambassador to Tokyo after Beijing sent a group of ships near a disputed island chain in the East China Sea.
The Japanese Coast Guard said eight Chinese maritime surveillance ships entered the disputed territory near the uninhabited islands early Tuesday.
It said this is the largest incursion by Chinese ships since tensions increased in September, when Japan purchased some of the islands from their private owners.
Since then, Beijing has regularly sent patrol boats and sometimes aircraft, in an attempt to challenge Japanese control of the strategic islands.
The situation Tuesday was complicated by the presence of a flotilla of Japanese activists who, with the escort of Japanese government ships, said they were to survey the islands.
China's State Oceanic Administration said three of its ships were on "regular patrol duty" in the area Tuesday, when they encountered several of the Japanese ships. It said five more Chinese ships were sent to the region in order to respond.
No clashes were reported. Reuters said the Japanese Coast Guard told the activists to leave the area once the Chinese ships came nearby. Tokyo summoned the Chinese ambassador to Japan following the incident.
Meanwhile, Japanese Prime Minister Shinzo Abe on Tuesday insisted the island chain remains under the "active control" of Tokyo. In a speech to lawmakers, he vowed to "expel by force" any Chinese landing on the islands.
"We have made sure that if there is an instance where there is an intrusion into our territory or it seems that there could be landing on the islands, then we will deal with it strongly," said Abe.
The islands, known as Senkaku in Japan and as Diaoyu in China, are surrounded by rich fishing grounds and thought to be located near untapped energy reserves. They are also claimed by Taiwan.
In recent months, Beijing and Tokyo also have sent airplanes to patrol the islands, raising concerns of an accidental clash between the two Asian powers.