A Hong Kong bookseller who went missing several days ago is thought to be in detention in mainland China.
The wife of Lee Bo said her husband had called her from a telephone number in Shenzhen to say he was helping with an investigation.
Mr Lee is the fifth person linked to the same bookshop to go missing in the past two months.
The case has raised fears that China is undermining Hong Kong's legal independence.
Demonstrators held a small rally outside Beijing's representative office in Hong Kong on Sunday to protest at Mr Lee's disappearance.
One of protesters, Raphael Wong, said: "Freedom of [a] person is inviolable. If the central government arrest a Hong Kong resident and [take them] back to mainland China, this is a threat to our freedom of press and also freedom of speech."
Meanwhile, a separate demonstration has been held in Hong Kong to protest at the appointment of a pro-Beijing official to the territory's main university.
Arthur Li was selected by the territory's leader, CY Leung, to chair Hong Kong University's governing council.
Protesting teachers and students view the appointment as a sign of growing political interference in the territory's education system.
Not back soon
Lee Bo went missing last Wednesday when he went to a warehouse in Hong Kong to collect a consignment of books.
The authorities in the territory do not appear to know where he is.
Carrie Lam, Hong Kong's chief secretary, said in a statement on Saturday that she was concerned about reports that the bookseller had gone missing.
The only clue to his whereabouts have come from his wife, Sophie Choi, who said she had received several short telephone calls from her husband.
"He said he wouldn't be back so soon and he was assisting in an investigation," she is reported to have told Hong Kong's Cable Television.
She said the telephone number of the calls suggested they were made in Shenzhen, a city on the Chinese mainland just across the border from Hong Kong.
Mr Lee is the fifth person associated with the Causeway Bay Bookstore to go missing.
Two of the other four men were last seen in Shenzhen, where their wives live; one was last seen in Hong Kong; and the other, the owner of a publishing house linked to the bookstore, was last heard from by email from Pattaya, Thailand, where he owns a holiday home.
Mr Lee spoke to the BBC when his four colleagues went missing, but was so worried about reprisals from China that he declined to give his full name.
There is no official word on why the five are missing, but the bookshop they are all linked to stocks publications critical of the Chinese government.
Chinese government critics on the mainland frequently disappear without a word, only to re-appear later in police detention.
But this is a new development for Hong Kong, which was allowed to keep its own political and legal system when Britain handed back the territory to China in 1997.
Some see these latest disappearances as a sign that China is using shadowy tactics to undermine Hong Kong's independence.
At a media conference, Democratic Party lawmaker Albert Ho said people in Hong Kong were shocked and appalled.
"We have strong reason to believe Mr Lee was kidnapped and smuggled back to the mainland for political investigation," he said.