Thousands of people took to the streets of Hong Kong on Sunday to protest at the mysterious disappearance of five booksellers amid suspicions of China's role in the abductions.

"This is a matter of utmost gravity and concern to Hong Kong," said lawmaker Alan Leong, who took part in the march that was aimed at putting pressure on Beijing to respond to allegations that the five booksellers had been kidnapped by mainland Chinese security forces from Thailand, Hong Kong and southern China.

An estimated 6,000 people took in the rally, the South China Morning Post reported.

"How can you say there is one country two systems if someone who has been practising his freedom of expression can be spirited away ... from right in the middle of the city," said Leong of the disappearance of Lee Bo.

Lee, a well-known bookseller who holds a British passport, was last seen on December 30. His bookshop was known for publishing books critical of mainland politicians.

But there was a twist last week, when Lee's wife withdrew her missing person's report to police saying he had been in contact. There is widespread speculation that she was forced to withdraw the statement. 

A video of Lee has also reportedly emerged at the weekend saying he was safe and asking Hong Kongers not to protest.

"I haven't seen the video," said Leong. "But based on what's been reported of the content, I personally read it as not a message that says Mr Lee is safe."

Apart from Lee, the others missing are Gui Minhai, Lui Por, Cheung Ji-ping and Lam Wing-kei.

All five men are linked to publisher Mighty Current, and the Causeway Bay bookstore, which publishes books that deal in unconfirmed rumours about high-ranking members of the Chinese Communist Party.

The disappearances have stoked fears in residents who read about secret detentions of high-profile individuals on the mainland, but have always believed themselves to be safe from such intrusions in Hong Kong. 

"They must abide by the laws of Hong Kong," the Hong Kong government said in a statement Sunday, while reiterating that they were pressing the mainland government for more information.

However, despite police making inquiries to mainland authorities about the first of the disappearances - that of Swedish national Gui Minhai from Thailand in October - there has been no response, said Leong.

"Not even a 'we don't know'," he said.

Chinese officials have not commented on the case, but state-run Global Times said that the bookshop was making a "profit by political provocation."

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