Japan arrested 14 people after pro-China activists landed Wednesday on an island at the centre of a bitter territorial row in an episode that threatens to further destabilise fractious ties.

The group -- some of whom made it to shore -- had sailed from Hong Kong on Sunday to the archipelago, known as Senkaku in Japan and Diaoyu in China, with the intention of planting a Chinese flag.

Prime Minister Yoshihiko Noda told reporters Japan would "deal with the incident strictly in line with the law", while the vice-foreign minister summoned the Chinese ambassador to lodge a complaint with Beijing.

Seven people jumped into the water from their boat, which had been surrounded by coastguard vessels and a helicopter, and reached the rocky shore at about 5:30 pm (0830 GMT), local police and coastguard officials said.

Two of the activists then returned to their boat, they said.

"The Okinawa prefectural police arrested five men for violation of the immigration control law on Uotsurijima," a police spokesman told AFP, referring to one of the islands in the archipelago.

Several hours later, a Japanese Coast Guard spokesman said: "We have also arrested nine more people on suspicion of illegally entering our country.

"We have now taken a total of 14 people into custody and will carry out a full investigation."

All those arrested were being sent to the Okinawa main island for investigation, reports said.

The arrests capped a day of diplomatic wrangling, as Asia marked the anniversary of Tokyo's World War II surrender.

China and South Korea demanded Japan do more to atone for the brutal expansionism of the 20th century, while in Tokyo cabinet ministers paid tribute to fallen Japanese, including war criminals, at a controversial shrine.

As well as festering resentment about past behaviour, territorial disputes have also come sharply into focus in the past week, after South Korean President Lee Myung-Bak's visit to another set of contested islands last week.

Jia Qingguo, professor at Peking University's School of International Studies, said leadership changes in China and South Korea were exacerbating age-old fissures.

A general election is due later this year in South Korea and Beijing is readying for a once-in-a-decade power transfer when President Hu Jintao and other leaders step down from their top Communist Party posts this autumn.

There was also a small protest in Taiwan against Japan, while North Korean state media said the Japanese ministers' visit to the Yasukuni Shrine was an "intolerable insult".

In Tokyo, Japan's vice-foreign minister Kenichiro Sasae called in the Chinese Ambassador Cheng Yonghua to "strongly protest" the pro-Beijing stunt, the foreign ministry said.

Cheng told Sasae that he would deliver the statement to Beijing immediately but expressed "China's own claim" over the islands.

Separately, China's vice-foreign minister Fu Ying summoned the Japanese ambassador to China, Uichiro Niwa, Xinhua news agency reported.

Fu also telephoned her Japanese counterpart Tsuyoshi Yamaguchi and reiterated China's sovereignty over the islands, demanding the Japanese guarantee the safety of those arrested and release them immediately.

The activists, who belong to the Action Committee for Defending the Diaoyu Islands, had said the move was aimed at countering a plan by a group of Japanese lawmakers to visit the disputed islands this weekend.

"We're very happy, we have tried many times and we declare this trip a big success," Action Committee for Defending the Diaoyu Islands spokesman Chan Yu-lam told reporters in Hong Kong.

"They (the Japanese) are like thieves, they take away the Diaoyu islands from us and they tried to stop us. They're like a thief shouting 'stop thief'," he said, using a Chinese proverb.

The activists made it to the island despite the Japan Coast Guard's heightened security, which included firing water cannon at their boat, said the protest group's leader.

Pro-China groups have made repeated attempts to reach the islands, but apart from successful forays in 1996 and in 2004, they have been blocked by Japanese patrol vessels.

Hong Kong leader Leung Chun-ying said he will "closely" monitor the incident and that the city's immigration officials will be in Japan to provide assistance to the activists.

"Japan must ensure the personal safety of the activists, especially the Hong Kong residents," he told reporters.

Despite the multiple claims for the territory, Japan will press ahead with a plan to purchase part of the disputed island chain, a senior official from the Tokyo metropolitan government was reported as saying by Kyodo news agency.

The uninhabited outcrops were the scene of a particularly nasty confrontation in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of coastguard vessels.

The United States meanwhile called on the "claimants to resolve the issue through peaceful means," with State Department spokeswoman Victoria Nuland adding: "Any kinds of provocations are not helpful in that regard."