Pic BBC Chinese Another associate of a Hong Kong bookshop specialising in titles critical of the Chinese government appears to have disappeared

He said that one possible explanation for the disappearances was that the publishing company was being pressured to scrap plans for an upcoming book rumoured to be about an old "girlfriend or mistress" of Chinese President Xi Jinping.

The five work for a publishing house known for producing books critical of the Chinese government.

"The government cares very much about Hong Kong residents" rights and safety", Leung Chun-ying told reporters, saying it would be "unacceptable' if mainland law enforcers were operating in Hong Kong.

Lawmaker and Labour Party chairman Lee Cheuk-yan said the idea that Chinese law enforcement could act of their own volition in Hong Kong was "terrifying". "The Hong Kong government is extremely anxious concerning the case".

Hong Kong, a former British colony, is an autonomous region of China. But she said the number from which Li was calling her did not belong to him and originated from the neighbouring mainland Chinese city of Shenzhen. The Chinese words on banner reads "Where is Lee Bo?"

He is the fifth missing person case related to the Causeway Bay bookshop since the owner Gui Minhai went missing while on holiday in Thailand in October.

Hong Kong enjoys press freedoms not available in China, although some publishers had already begun to complain about growing pressure from Beijing. But Chan said chief executive Leung should be taking the matter up at the highest level in Beijing.

Lee told her he was assisting in an investigation and he would not return home for a period of time.

Four of his colleagues from the Mighty Current publishing house went missing in October.

On Sunday, a group of lawmakers and activists marched to the central Chinese government's liaison office in Hong Kong, demanding answers about the missing people.

The booksellers specialised in publications critical of the Communist Party government.

Mr Leung noted widespread rumours that mainland officers are behind the sudden vanishing of Mr Lee Bo, a shareholder in publisher Mighty Current which puts out salacious and politically sensitive tomes about China's leaders.

The Hong Kong authorities later also confirmed that Lee did not have any record of having exited the island.

"Chinese authorities should investigate these reported disappearances and immediately clarify the situation".

Hua Chunying, China's foreign ministry spokesperson, told journalists at a regularly-scheduled press conference that she had no information about the case. But it simultaneously launched an attack on the publishers. "These books spread to the mainland by various means, becoming a source of political rumours, and creating negative effects", said the article, signed by Shan Renping, a pen name for the newspaper's editor Hu Xijin.

Causeway Bay Books has remained closed following the incidents.

However, there is "no indication" as of yet that Chinese authorities had any role to play in the disappearance of Lee or his colleagues, Leung said Monday.

Large crowds of protesters also gathered in Hong Kong after a pro-Beijing official was given a senior role at the main university. "We are deeply concerned by reports about the disappearance and detention of individuals associated with the Causeway Bay Books bookstore in Hong Kong".