The UK should not interfere in Chinese affairs, observers said Saturday, after a British government report claimed that Hong Kong publisher Lee Bo had been "involuntarily" taken to the Chinese mainland, a claim which China has strenuously denied.

Foreign ministry spokesperson Hong Lei said in a Friday statement that China is strongly displeased with and firmly opposed to the UK government report on Hong Kong, which accused the mainland of infringement upon Hong Kong's freedom.

"Mr. Lee was involuntarily removed to the mainland without any due process under Hong Kong SAR law," the report said, adding that it is "a serious breach of the Sino-British Joint Declaration on Hong Kong and undermines the principle of "One Country, Two Systems."

Hong Kong is a special administrative region of China, and Hong Kong affairs are China's domestic affairs, Hong said, stressing that no foreign country has the right to interfere. He urged the British side to be cautious with words and deeds and stop interfering in Hong Kong affairs.

"The UK should get its facts straight before making groundless remarks, and it could express its concerns to the Chinese government via diplomatic channels instead of releasing such reports," Zhang Dinghuai, a professor at the Contemporary Chinese Politics Research Institute of Shenzhen University, told the Global Times.

No grounds 

Lee, a shareholder in the Mighty Current publishing house, allegedly went missing on December 30 from Hong Kong. It was alleged that four other people associated with Mighty Current were also missing, with the case triggering strong protests in Hong Kong. 

It was later revealed that Lee had traveled voluntarily to the mainland and was cooperating with investigators. He asked Hong Kong police to stop their investigation, Hong Kong-based Sing Tao Daily reported. 

"There are no grounds to say there is a serious breach of the Sino-British Joint Declaration on Hong Kong, as this bilateral agreement mainly deals with China's resumed sovereignty over Hong Kong, but Lee's case is an internal matter that should not be dealt with under international treaties," said Jiao Hongchang, deputy dean of the Law School at the China University of Political Science and Law. 

Since July 1997, the British Foreign Secretary has reported to Parliament at six-month intervals on the implementation of the Sino-British Joint Declaration on the Question of Hong Kong. The latest report issued Thursday by Philip Hammond covered July 1 to December 31, 2015, according to the Xinhua News Agency.

"The UK should also avoid such groundless judgments in order to safeguard the stable ties between China and the UK, especially since Chinese President Xi Jinping's State visit to the UK in last September heralded a golden era for the bilateral relationship," said Chen Lijun, a professor from the Center for Studies of Hong Kong, Macao and Pearl River Delta under Sun Yat-Sen University.

Volatile city

Hong Kong has often been volatile in recent years, following the Occupy Central protests of 2014 and rallies targeting mainland parallel traders in 2015.

On Tuesday, a riot erupted in Mong Kok shopping district after police tried to clear away street vendors, with mobs barricading the street, setting fires, damaging police cars, and assaulting police officers, injuring 89 police officers and several journalists. 

Early on Saturday, 31 vehicles in a parking lot at Lai Chi Kok container port were set on fire. Police said the fire, which started from two containers filled with styrofoam at midnight Friday, may be arson, China Central Television reported.

A slowing economy and the gap between the rich and poor may be to blame for many of these disturbances, experts said.

"The root of those riots lies in the economic imbalance of different classes in Hong Kong and the lack of mobility. It's especially hard for the young generation to move up," Zhu Shihai, a professor specializing in the study of Hong Kong at the Macao University of Science and Technology, told the Global Times.

Beijing is obliged to take measures to adjust the economic imbalance and address social contradictions in Hong Kong to safeguard the interests of the lower-income labor force and of the unemployed, analysts said.

"The mainland can fully utilize the region's free-trade environment to help boost its economy and benefit local residents," Zhang said, adding that the UK could also invest more in Hong Kong, rather than interfering. 

Xinhua and Wu Zhiwei contributed to this story