The Chinese foreign ministry on Sunday urged Japan to stop any action that undermines China's territorial sovereignty after 10 Japanese activists landed on the Diaoyu Islands, which further inflamed tensions between the two Asian powers.

The ministry has lodged a solemn representation with the Japanese ambassador to China to strongly protest against Japanese rightists' illegal acts, said ministry spokesperson Qin Gang in a statement.

A total of 10 activists swam ashore on Sunday morning and planted several Japanese flags after the 20-boat flotilla arrived at the waters near the Diaoyu Islands, the Kyodo News agency reported.

The group did not include any parliamentarians, but some local politicians set foot on the island, according to the report.

A fleet of around 150 rightists, including eight parliamentarians, departed from Ishigaki city Saturday and arrived at the waters early Sunday morning. Their earlier landing application had been rejected by the Japanese government.

"The move by the Japanese rightists is obviously retaliation against the landing by a group of Chinese people days ago," Zhou Yongsheng, a professor of Japanese affairs at the China Foreign Affairs University, told the Global Times.

The Chinese activists who landed on the Diaoyu Islands on Wednesday said earlier that they made the latest trip in order to assert sovereignty after Japanese right wing politicians planned to land on the islands and talked about "buying" the islands from private "owners," moves many analysts deemed provocative.

Fourteen activists were detained by the Japanese Coast Guard (JCG) after seven of them landed to assert China's territorial claim. Seven of the detainees returned to Hong Kong on Friday evening.

The Japanese activists' arrival on the Diaoyu Islands sparked massive protests in a number of Chinese cities. People from Guangzhou, Shenzhen, Hangzhou, Shenyang, Harbin and Qingdao, among others, took to the streets on Sunday to voice their dissatisfaction, the Xinhua News Agency reported.

Qiu Hanbang, a Shenzhen resident who participated in the protest, told the Global Times that the city saw an estimated 4,000 demonstrators on Sunday, who held various banners while chanting slogans and singing the national anthem.

"I think it's a significant activity which will let other countries, especially Japan, know that we Chinese are not meat on the chopping block," said Qiu. "It's also a good opportunity for the younger generation to feel patriotism."

But some demonstrators went to further extremes. Pictures circulating online showed that some Japan-made cars were smashed, which drew massive public criticism of hooliganism in the name of patriotism.

Qiu admitted that some people threw water bottles at passing Japan-made cars, which he described as "irrational."

Meanwhile, debates on how to solve the feud between the two countries are also heating up among scholars. At a forum on the Diaoyu Islands hosted by the Global Times Sunday, scores of experts shared their views.

Xu Sen'an, a researcher with the State Oceanic Administration, said China should make no concession regarding territorial issues, adding that multiple measures including political, economic and military ones should be adopted to make Japan realize its fault.

Luo Yuan, major general of the PLA Academy of Military Sciences,  said he believes that China has to clearly set out red lines that Japan should not pass.

"Such actions toward the islands by Japan as landing, nationalization and establishing garrisons should be absolutely forbidden," said Luo, adding that a Chinese coastguard force should be set up as soon as possible.

Japan plans to replace its ambassador to China, Uichiro Niwa, possibly in October, the Yomiuri Shimbun newspaper reported Sunday. Niwa earlier opposed the Tokyo metropolitan government's plan to purchase the Diaoyu Islands.

The Sankei Shimbun newspaper reported that Japan and the US will launch a joint military drill on Tuesday, which aims to practice how Japan would respond if the Diaoyu Islands were "invaded by the Chinese army."