Activists are protesting Hong Kong's disqualification of a legislative candidate on the grounds that she advocated self-determination for the Chinese territory.
Lau Siu-lai, who was stripped of her seat in the Hong Kong legislature last year, was barred by the government on Friday from running in a November 25 election.
Hong Kong authorities are trying to quash pro-independence voices ahead of the election.
Chinese President Xi Jinping and other officials have warned separatist activity will not be tolerated.
The pro-independence Hong Kong National Party was banned in September by authorities who invoked a 1997 national security law for the first time.
Pro-democracy activists complain Beijing interferes in Hong Kong's affairs in violation of its promise of a "high degree of autonomy" when the former British colony returned to Chinese rule in 1997.
Some have called for more autonomy or outright independence for this prosperous business center of 7.5 million people.
Authorities said Lau's statements in 2016 in support of self-determination were illegal. Lau dropped that call from her election platform, but the Election Affairs Commission said her views hadn't really changed.
The decision "amounts to the political screening of candidates Beijing does not like," said Hong Kong Watch, a human rights group, in a statement.
"The decision is in breach of the right to stand in free and fair elections," the group said.
A group of pro-democracy legislators said in a statement that the government is "trying to use fear against dissident voices."
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