BEIJING  - China demanded Saturday that Japan cease actions ‘harming’ its territorial sovereignty as a flotilla of boats carrying Japanese nationalists and lawmakers set sail for islands at the heart of a vitriolic diplomatic row with Beijing.

Around 150 people, including eight parliamentarians, left far southwest Ishigaki bound for the archipelago in the East China Sea, a day after Japan deported pro-China activists who had sailed there from Hong Kong. The foreign ministry statement was in response to plans by Japanese lawmakers and nationalists visiting the East China Sea islands. “China has made solemn representations to Japan, demanding that it immediately cease actions harming China’s territorial sovereignty,” said the statement, which was in response to a media query on the planned Japanese trip.

“China reiterates that any unilateral action taken by Japan regarding” the islands “are illegal and invalid”, it said, adding that any such actions will not undermine its claim over the territory.

It follows another statement late Friday which called on Japan to pursue “dialogue and negotiation” to resolve the dispute. Japan, which controls the uninhabited islands, on Friday deported 14 activists who had sailed there, moving swiftly to put an end to a potentially damaging row with Beijing.

Some of them became the first non-Japanese to set foot on any part of the archipelago - known as Diaoyu in China and Senkaku in Japan — since 2004. But Japanese Prime Minister Yoshihiko Noda has come under fire from some conservative lawmakers - including members of the main opposition Liberal Democratic Party and Tokyo’s nationalist governor Shintaro Ishihara - for not allowing the activists to be held for prosecution.

The long-running dispute flares up from time to time and has proven a stumbling block - along with issues related to Japan’s military occupation of parts of China during World War II - to smooth relations between Asia’s two biggest economies.

Separately, a Japanese ruling party heavyweight said Saturday that the country should beef up its coast guard to defend the islands.

“Coast guard officials are doing their best, and so the government and the ruling parties will discuss how to strengthen our backup to them,” Seiji Maehara, the policy chief of the Democratic Party of Japan told reporters.

The renewed dispute comes as tensions also spiked between Japan and South Korea after President Lee Myung-Bak visited islets controlled by Seoul but claimed by Tokyo.

Japan also has a separate territorial dispute with Russia over northern islands seized by the Soviet Union in the waning days of World War II.