Local media said six activists were wanted on suspicion of secession or colluding with foreign forces, crimes that the new law punishes with up to life in prison.
Disqualified candidates included activist Joshua Wong, some members of the Civic Party and others who won an unofficial primary vote held by the opposition camp earlier this month.
Beijing says French politicians were being misled by a slanderous American, Australian and British campaign while threatening to invalidate British National Overseas passports held by Hong Kong residents.
President Trump orders end to Hong Kong's special status under US law to punish China for what he called "oppressive actions" against former British colony.
More than 200,000 voters took part in the election to select candidates who stand the best chance of success in Legislative Council elections in September.
Hong Kong leader Carrie Lam hailed the opening as "a historic moment" and said China's intelligence apparatus would be an "important partner" in helping to safeguard national security.
TikTok to exit Hong Kong market within days, while other technology companies, including Facebook, WhatsApp, Telegram, Google, Zoom and Twitter, suspend processing government requests for user data in the region over Beijing's new security laws.
Last week, China enacted a security law outlawing four national security crimes: subversion, secession, terrorism and colluding with foreign forces.
Hong Kong public libraries have placed certain books under review to see whether they "violate the stipulations" of a new national security law that China imposed on the financial hub.
Police arrest more than 300 people as protesters hit streets in defiance of sweeping security law introduced by China in the semi-autonomous city.
Western governments and critics have warned the new law will curb the city's freedoms and undermine its "One Country, Two Systems" governance scheme, which technically allows freedoms unseen on the mainland.
The Civil Human Rights Front said the force had rejected its applications for rallies on July 1, the 23rd anniversary of the former British colony's handover to China.
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