Lam, one of five Hong Kong booksellers detained for months last year by China, had been due to lead Friday's protest march. Organizers said he had "suddenly" backed out.

He had returned to Hong Kong last month and described his eight-month ordeal in defiance of previous Chinese conditions set for his release.

His depiction fueled fears that Beijing was eroding the area's semi-autonomy, enshrined in 1997 when Britain ceded control of its former colony.

Albert Ho, a pro-democracy lawmaker who has been assisting Lam, said Lam noticed he had been followed by strangers the last two days.

"He feels increasingly concerned about his own personal safety so he made up his mind and decided not to attend the July 1st march," Ho said.

Leung effigy

As Friday's rally began, dozens of demonstrators had repeated call for Hong Kong's unpopular Chief Executive Leung Chun-Ying to resign, burning an effigy of him, according to the news agency AFP.

Police, expected to number 1,700 along the protest route, had already warned that they would take "resolute" action against "illegal acts."

Some activists have demanded a return to British rule as a stepping stone to independence in the wake of pro-democracy rallies in 2014 that failed to win concessions and left the protest movement fatigued.

Political scientist Ma Ngok of the Chinese University of Hong Kong said the so-called "localists" had been under pressure since rioting in the commercial neighborhood of Mong Kok in February.

At an official handover celebration Friday, Leung urged Hongkongers to be "united."

ipj/msh (AFP, AP)