China is accused of tightening its grip in semi-autonomous Hong Kong and is extremely sensitive to challenges from areas it considers to be part of its territory, including self-ruling Taiwan which it sees as a renegade province to be brought back into its fold.
Taiwan and Hong Kong are thorns in Beijing's side -- both saw huge anti-China protests in 2014, known respectively as the Sunflower Movement and Umbrella Movement.
Activists Joshua Wong, Nathan Law and Alex Chow were jailed for between six and eight months Thursday for their role in the Umbrella Movement, which called for fully free leadership elections for Hong Kong.
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Taiwan's top policy making body on China, the Mainland Affairs Council, expressed its "deep regret" over their imprisonment.
"The council reiterates the government's long-standing stance to support Hong Kong people to pursue democracy, freedom, the rule of law and human rights," it said in a statement.
It added the verdict had prompted concerns in Hong Kong and the international community over what some see as a politicised judiciary that would damage the city's judicial independence and affect its investment environment.
The ruling Democratic Progressive Party (DPP) also urged the Hong Kong government to look "positively" at calls for reform.
"Graver suppression will only spark more resistance," the party said in a statement.
Ties between Beijing and Taiwan have worsened since President Tsai Ing-wen of the DPP took office last year as she has refused to acknowledge the island is part of "one China".
Chinese authorities have also become increasingly incensed with Hong Kong as some campaigners call for the city to split completely from China.
Wong and Law attended the launch of a new group called the "Taiwan Congressional Hong Kong Caucus" in June, comprising 18 Taiwanese lawmakers who say they wanted to help promote democracy in Hong Kong.
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The lawmakers include Huang Kuo-chang, a former protest leader in Taiwan and now head of the New Power Party, which said in a statement Thursday that Hong Kong's democracy and rule of law were "facing extinction".
Huang was at the forefront of the Sunflower Movement, which occupied Taiwan's parliament in protest over trade deals with China, reflecting growing anti-Beijing sentiment.
A Taiwan court acquitted Huang and 21 other protesters over their roles in the demonstrations in March this year.
He and other top activists have been barred from entering Hong Kong since 2014.
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