Beijing's decision to restrict who can run to be Hong Kong's leader was closely watched by civic groups across the Taiwan Strait, where ties with some Hong Kong political factions have strengthened.

The decision "should serve as a red flag for Taiwan that Beijing could also break its promises to Taiwan no matter how rosy cross-strait ties appear right now," said Wu Jieh-min, a researcher at Academia Sinica's Institute of Sociology.

In recent years, Beijing has used a two-pronged approach to Taiwan—which broke away from the mainland in the 1949 Communist revolution and which Beijing still views to be part of its territory. It has offered economic trade advantages and investment incentives to Taiwanese businesses while stepping up pressure on the government to engage with Beijing in a political dialogue.

The local business community supports closer trade ties with China, but the Taiwan public remains largely suspicious of Beijing's intentions, fearing it is preparing to reclaim the island through any means necessary, including the use of force.

"If China is successful in strong-arming Hong Kong into submission, it would further embolden China to use the same tactic on Taiwan in the future," said Chen Yan-ming, a 53-year-old businessman in the food industry.

The Mainland Affairs Council, Taiwan's government body in charge of cross-strait affairs, expressed regret Sunday that Beijing's decision fell short of the Hong Kong people's expectation for universal suffrage, according to the Central News Agency. The council also urged Hong Kong and the mainland government to forge a consensus through "wisdom and patience."

Hong Kong groups participated in Taiwan's "Sunflower Movement" in March in which hundreds of thousands of people protested a trade deal with mainland China. Some Taiwan activists also took part in Hong Kong's pro-democracy rallies over the summer after China issued a policy statement reinforcing that Beijing holds ultimate control over Hong Kong.

In Taiwan, several groups who advocate formal independence from China issued a joint statement in July expressing their support for Hong Kong activists' pursuit of full democracy.

Lin Fei-fan, a student leader of the Sunflower Movement, wrote on his Facebook page that he wished he could stand "shoulder-to-shoulder" with the people in Hong Kong.

"The hope lies with the people, and changes spring from [civil] disobedience," he wrote.

Benny Tai, one of the co-founders of the pro-democracy Occupy Central movement in Hong Kong, urged Taiwan to be on guard with Beijing.

"Beijing's blatant rejection of our demand should show the Taiwanese people that they should not to have any expectations for Beijing's promise of 'one country-two systems,'" Mr. Tai said on Sunday.

But despite growing collaboration, Leung Man-to, a political-science professor at Taiwan's National Cheng Kung University, said, "It remains to be seen how the Taiwanese people would really react once Beijing steps up political pressure, as Beijing has taken a different and more grass-root approach to win Taiwan over versus the more top-down, heavy-handed approach seen in Hong Kong."

Mr. Leung said that although activists on both sides take a strong interest in the issue, many people in Taiwan care little about the prospect of growing Beijing influence either in Hong Kong or Taiwan.

Indeed, for many young Taiwanese, Hong Kong represents a place to go shopping rather than a political hotspot.

"When I visited Hong Kong last summer, I didn't feel that I was in a place with no freedom," said Cathy Young, 18, a Taipei university student. "But then again, I don't really care about politics. I just care that my right to pursue my dreams, get a good job, and live my life the way I choose will not be taken away."