Four Japanese citizens, including local politicians, landed on disputed islands in the East China Sea on Tuesday, drawing an official protest from Beijing.

The group sailed to the uninhabited islets, known as Senkaku in Japan and Diaoyu in China, on a fishing boat and stayed there for more than two hours, according to the Japan Coast Guard.

Beijing said it had "raised its solemn representations and protest" with Tokyo over the move, which came as Japan and China commemorate the 40th anniversary of normalisation of diplomatic ties this year.

"China's determination to safeguard its sovereignty of the Diaoyu islands is unswerving," foreign ministry spokesman Hong Lei said in a statement in which he reiterated Beijing's "indisputable sovereignty" over the chain of islands.

The foreign ministry also confirmed it had summoned a "diplomatic envoy" from the Japanese embassy in Beijing, without specifying who.

The tiny isles, also claimed by Taiwan which said it would lodge a protest with Tokyo, have been a source of tension between Japan and China for decades.

When Japanese Prime Minister Yoshihiko Noda visited China on December 25-26, he agreed with Beijing to set up a high-level meeting on maritime affairs in an effort to reduce tensions.

A dozen pro-China activists also set sail from Hong Kong for the disputed islands but were halted by marine police who "had grounds to believe that the vessel would not be used for fishing," Hong Kong Marine Department said.

"Diaoyu island is ours. It is from our ancestors. We are protesting Japan's attitude and actions," their spokesman Huang Hsi-lin told reporters before they departed on their abortive trip.

The pro-China group, including activists from Taiwan and Hong Kong, has made repeated attempts to land on the islands, but apart from one successful foray in 1996 they have been blocked by Japanese patrol vessels.

The Japanese government leases the islands from private land owners, and bans entry in a bid to prevent political incidents.

The Japanese coast guard said that three citizens on board the fishing boat, which sailed from Ishigaki in Japan's southwest, landed Tuesday on one of the islands around 9:30 am (0030 GMT) while the fourth followed 20 minutes later.

Of the four, two were identified as Hitoshi Nakama and Tadashi Nakamine of the Ishigaki municipal assembly, while the identities of the other two were not immediately known, a spokesman said.

Nakama also went to the isles in 2010 without the Japanese central government's permission.

"The coast guard's patrol vessels are always near the Senkaku islands. As their boat approached Senkaku, the coast guard verbally instructed them not to go any closer," the spokesman said.

"We have told them that it was against the law to land on the island. They said they were going fishing. We are not in position to stop them from going to fishing."

The four men returned to the boat before noon and began their return trip to Ishigaki, the coast guard said.

The pro-China activists who set sail from Hong Kong said it was the group's first trip from the territory after previous attempts staged from Taiwan were blocked.

"Apart from the first year, we have had no success reaching the islands because of attempts to block us. We have been continually harassed by the Japanese side," Huang said before they were stopped by Hong Kong police.

"Every year it is the same, Japanese boats will surround our boat and stop us from approaching the islands."

In September 2010, relations between Tokyo and Beijing turned icy after a collision between a Japanese coast guard vessel and a Chinese fishing boat off the isles which are administered by Tokyo.

After Japan arrested the skipper, a furious China demanded his immediate release, tightening trade restrictions and cancelling diplomatic, political and cultural exchanges.

Japan eventually released the fisherman, but the incident fueled anti-China sentiment among the Japanese public.

burs/mtp/emb