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.
Earlier, New Zealand suspended its extradition treaty with Hong Kong, joining Canada, Australia and the UK which have already6 made the same moves in response to China's national security law on the territory.
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.
China slammed the US move, which came one day after the unveiling of a US indictment targeting two Chinese nationals for allegedly hacking hundreds of companies worldwide and seeking to steal virus vaccine research.
Top US diplomat Mike Pompeo offers unsparing criticism of the Chinese Communist Party while visiting London and pledges to build a global alliance against Beijing.
Anti-government protesters have adapted new methods to protest without violating the new security law legislation.
Leader Carrie Lam says a third of all infections since the outbreak started have been recorded in the last fortnight. HSBC, Standard Chartered and other banks are racing to close branches or curtail working hours after the spike in cases.
"Hong Kong will now be treated the same as mainland China – no special privilege," US President Donald Trump earlier said in a statement, authorising sanctions on banks over China's clampdown in the financial hub.
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.
On the eve of the primaries, police searched the office of independent pollster Robert Chung, whose Hong Kong Public Opinion Research Institute helps organise the election, raising concerns among activists of interference in the poll.
Police said the seven arrested – five men and two women – could face up to 10 years in prison for "assisting" a man suspected of stabbing a police officer during a protest against a new national security law on July 1.
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