A Hong Kong bookseller who was set to lead the southern Chinese city's annual pro-democracy protest march has dropped out at the last minute, saying he felt "gravely threatened."

Organizers of the protest, which is expected to attract tens of thousands of people, had invited Lam Wing-kee. But they said in a message to the media that he "suddenly" backed out Friday.

Lam is one of five booksellers who went missing for months only to turn up later in police custody in mainland China.

Their disappearance sparked international concern that Beijing was eroding Hong Kong's considerable autonomy and rule of law.

Lam returned to Hong Kong last month but defied Chinese authorities to speak out about his ordeal of being detained secretly on the mainland.