One of the men in the flotilla displayed a Japanese flag
The Chinese government has warned Japan to stop an "illegal" trip by politicians to a group of disputed islands under Japanese control.
A flotilla of some 20 Japanese boats has set sail for the Senkaku (Chinese: Diaoyu) islands and is expected to anchor off them early on Sunday.
The politicians plan to commemorate Japanese dead in World War II, when Japan occupied eastern China.
But Japan's government has denied them permission to land on the islands.
China says the event will undermine its "territorial sovereignty" and this is the latest move in an escalating dispute over the islands.
On Friday, Japan deported several Chinese activists who had landed there this week.
The islands, also claimed by Taiwan, are close to strategically important shipping lanes, offer rich fishing grounds and are thought to contain oil deposits.
Emotions have been running high since the commemoration on Wednesday of Japan's surrender in World War II, when China and South Korea both protested against a visit to a Tokyo war shrine by two Japanese cabinet members.
Just before 21:00 (12:00 GMT), the 150-strong party sailed out of the Japanese port of Ishigaki.
They are expected to arrive off the disputed islands in the East China Sea at dawn on Sunday.
"I want to show the international community that these islands are ours," Kenichi Kojima, a local politician from Kanagawa, near Tokyo, told AFP news agency before he boarded.
"It is Japan's future at stake."
Chinese foreign ministry spokesman Qin Gan said: "Any unilateral action taken by Japan on the Diaoyu Islands is illegal and invalid."
Earlier this week, activists sailed to the disputed island chain from Hong Kong in a protest aimed at promoting Chinese sovereignty.
China had praised Japan's "wise" decision to free them, saying in an article on Xinhua news agency's website that the speedy action had averted a deterioration in relations.
Rows over the disputed islands have caused Sino-Japanese ties to freeze in the past.
China claims the largely uninhabited islands has been a part of its territory since ancient times but Japan says it took control of the archipelago in the late 1890s after making sure they were uninhabited.
In September 2010, relations plummeted after the arrest of a Chinese trawler captain near the islands.
The captain was accused of ramming two Japanese patrol vessels in the area, but Japan eventually dropped the charges against him.