HONG KONG (AP) — Hong Kong pro-democracy groups rallied Friday 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 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 turned up in mainland police custody.

Lam said he was detained after crossing Hong Kong's border with mainland China, blindfolded for a 13-hour train ride to a city near Shanghai and confined for months to a room, where he was interrogated by mainland authorities.

He said his interrogators wanted details of the buyers and authors of his company's books, which were popular with Chinese visitors to Hong Kong but banned in the mainland.

Protesters from Demosisto, a small political party run by young activists, tossed newspapers with stories about the case, a banned book and a petition letter over the liaison office's fence. They carried placards that said, "No cross-border abduction."

Lam "risked his life to tell the truth and he risked his life to protect the values of Hong Kong people," said Nathan Law, Demosisto's president. "He somehow united all the Hong Kong people and we realized that the dirty hand of the tyrants is getting closer and every one of us is at risk."

Two other pro-democracy parties held separate protests in front of the liaison office.