A Hong Kong bookseller's revelation of months spent in harrowing detention by mainland Chinese authorities is inflaming tense relations between the semiautonomous city and Beijing, with pro-democracy activists staging protests Friday.

Lam Wing-kee's account to reporters a day earlier directly contradicted the official version of events surrounding the disappearance of him and four other men linked to a Hong Kong publisher of banned books on China's Communist leadership.

His detailed testimony supports widespread suspicions that the five were seized by Beijing authorities as part of a campaign to silence critical voices, and had not willingly traveled to mainland China to voluntarily admit to crimes or help with investigations, as they had previously stated on Chinese television.

The saga of the missing booksellers underscores growing fear in Hong Kong that Beijing is tightening its hold on the city and eroding its considerable autonomy.

Activists are particularly concerned about that the mainland's security agencies may be extending their reach, as The Christian Science Monitor reported in January.

"This phenomenon illustrates that China's dictatorial rule is expanding," Chinese legal scholar and rights lawyer Teng Biao told the Monitor. "With its economic prowess, it can increasingly ignore the international community and international law."

China's Communist government took over control of Hong Kong from Britain in 1997, promising to let it retain civil liberties such as freedom of speech for 50 years under a system known as "one country, two systems."

The case "will make the people of Hong Kong feel unsafe and there will be a blow to the already fragile one country, two systems" framework, said Zhang Lifan, a political commentator in Beijing. "The Hong Kong public will no longer believe what (the government) says in the future and it may result in a public trust crisis."

In Beijing, Ministry of Foreign Affairs spokeswoman Hua Chunying said "China is unswervingly determined to implement the policy of 'one country, two systems.'" She told a regular news briefing that Lam is a Chinese citizen and "violated Chinese laws in mainland China, thus the competent authorities in China certainly have the rights to deal with it in accordance with law."

Public discontent has risen sharply in recent years over mainland China's rising influence in Hong Kong. In 2014, activists brought key intersections to a standstill for 79 days to protest Beijing's decision to restrict elections for the city's top leader. The protests ended when Hong Kong's Beijing-backed leader refused to make concessions, but they spawned a new wave of radical activist groups campaigning against the disappearance of Hong Kong's Cantonese culture and advocating its independence from China.

Some of these groups plan to field candidates against both pro-Beijing rivals and moderate established pro-democratic parties in citywide legislative elections set for September, which threatens to further polarize the city.

"There's really a need for mainland officials to examine their policies and think about how over the last couple years their hard line on democracy and increasing interference in Hong Kong has stirred up a lot of opposition," said Michael Davis, a law professor and constitutional affairs expert at Hong Kong University.

In Hong Kong on Friday, three pro-democracy political parties held separate rallies in front of Beijing's liaison office to vent their anger.

Protesters from Demosisto, a small, newly formed political party run by young people including teen activist Joshua Wong, tossed newspapers with front-page 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."

The disappearances also shocked the city because one of the men, British citizen Lee Bo, is suspected of being abducted to the mainland by Chinese security agents operating in Hong Kong, which is prohibited by Hong Kong's mini-constitution. Lam said Lee confirmed this to him on Thursday, contradicting Lee's earlier statements that he made his own way to the mainland.

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 five months in a small room, where he was kept under surveillance and interrogated.

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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.

He was forced to sign a confession that was used as a script when he went on a Chinese TV channel to say he broke the law by mailing his company's books to the mainland.