The disappearances have prompted fears that mainland Chinese authorities may be using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong has been governed since its return to China from British rule in 1997.
Chinese police confirmed for the first time on Thursday that three of the five Hong Kong booksellers who went missing were being investigated for "illegal activities" in China.
The booksellers specialize in selling and publishing gossipy political books on China's Communist Party leaders.
The news came two weeks after their colleague, Swedish national Gui Minhai, who disappeared from Thailand in October, appeared on Chinese state television and made a tearful confession to a drink-driving offence more than a decade ago.
Shortly after, Chinese authorities confirmed that Lee Bo, 65, one of the other missing booksellers and a British passport holder, was in China after he vanished from Hong Kong weeks earlier.
Their mysterious disappearances had sparked fears they may have been taken by Chinese agents.
"The resolution calls for their immediate safe release. It also calls for the immediate release of all other persons arbitrarily arrested for exercising their rights to freedom of expression and publication in Hong Kong," the European Parliament said in a statement.
Asked about the statement, Chinese Foreign Ministry spokesman Lu Kang told a daily news briefing: "Hong Kong affairs are China's domestic matter. We firmly oppose any country interfering in China's domestic affairs."
So far, Chinese authorities have not made any substantial statements explaining Beijing's role in the disappearances, nor how the men ended up in China.
Lu said earlier this week that Chinese law enforcers always followed the law and would never do anything illegal, especially not overseas.
The "one country, two systems" formula accords Hong Kong a degree of autonomy and freedoms not enjoyed in mainland China, including freedom of speech.
The British government is still waiting for responses to its diplomatic requests for information and access to Lee.
Authorities including in the European Union and the United States have expressed concerns over the disappearances.
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