Lee Bo, 65, a major shareholder of Causeway Bay Books, "vanished" on Wednesday after he went to fetch books from his warehouse in the city, Lee's wife told Hong Kong media.
She said her husband had called her from a mainland Chinese number to tell her he was safe but would not reveal his location, the South China Morning Post (SCMP) reported on Sunday. She said Lee spoke to her in Mandarin even though the pair usually communicated in Cantonese.
Lee is the fifth person linked to the bookstore to have gone missing in the past two months. Others include Gui Minhai, owner of Mighty Current, the publishing house that owns the bookstore, the SCMP reported. Missing person reports were filed for three others, it said.
The disappearances have stoked fears of mainland Chinese authorities using shadowy tactics that erode the one country, two systems formula under which Hong Kong has been governed since its return to Chinese rule from Britain in 1997. The city's constitution guarantees wide-ranging personal freedoms and independent law enforcement.
"We have strong reason to believe Mr Lee was kidnapped and smuggled back to the mainland for political investigation."
Beijing's Liaison Office and the Hong Kong Immigration Department could not be reached for comment. The Hong Kong and Macau Affairs Office, an agency of China's State Council, did not immediately respond to a request for comment.
About 60 people marched to the Liaison Office in the financial hub, while pro-democracy lawmakers held a media conference to express concern over the disappearance.
"We have strong reason to believe Mr Lee was kidnapped and smuggled back to the mainland for political investigation," said Democratic Party lawmaker Albert Ho.
He did not elaborate on the reason, but the group called on the Hong Kong government to say whether there was an official immigration departure record for Lee.
Secretary for Security, John Lee, said the police would be expanding the scope of its investigation into Lee's disappearance, although it was not possible to give details about every line of inquiry, broadcaster RTHK reported on Sunday.
The Causeway Bay Bookstore sells paperbacks highly critical of the Chinese leadership in Beijing and often containing details of the private lives of senior leaders and their families.
While the books are banned in mainland China, local media said they are popular with Chinese tourists in Hong Kong.
Hong Kong's second highest-ranking official, Chief Secretary Carrie Lam, expressed concern about the case on Saturday.
"The Hong Kong government is highly concerned about the wellbeing of Hong Kong people in the city and abroad. I know the police are already working on the case," Lam said.