Hong Kong pro-democracy groups are rallying against China's tightening grip on the semiautonomous city following electrifying revelations by a bookseller who spoke out about his months-long detention in the mainland.

Several dozen people marched Friday morning to Beijing's liaison office to vent their anger after Lam Wing-kee's testimony about his harrowing ordeal.

He's one of five people linked to a publisher specializing in salacious books on elite Chinese politics. They disappeared last year and later ended up in mainland police custody.

Lam said he was blindfolded, detained and interrogated for months by mainland authorities.

Protesters from the Demosisto political party tossed a newspaper, a book and a petition letter over the liaison office's fence.

An organizer said Lam "risked his life to tell the truth."