The seven were arrested Tuesday and Wednesday for "offences in connection with seditious intention" in relation to social media posts commemorating Beijing's 1989 Tiananmen Square crackdown.
They were the first people to be arrested under the "Safeguarding National Security Ordinance" -- enacted by Hong Kong in March and commonly referred to as Article 23 -- which penalises sedition with up to seven years in prison.
The European Union had said the arrests suggest that the new legislation "is used to stifle freedom of expression".
On Thursday, China's foreign ministry in Hong Kong hit out at international critics.
"We advise individual countries and politicians to face reality squarely, uphold an objective and impartial stance... and stop interfering in Hong Kong's affairs and China's internal affairs immediately," said a ministry spokesperson in the city.
Hong Kong's security chief said that one of those arrested was Chow Hang-tung, a prominent activist who led the now-disbanded group that once organised annual vigils to mark the Tiananmen Square crackdown.
Chow is already serving a more than 30-month jail sentence over other charges, including "unauthorised assembly" for her attempt to publicly commemorate June 4.
- EU, US criticism -
The Chinese foreign ministry office said Thursday that Chow "continuously published posts with seditious intent on social media, trying to provoke hatred of the public towards the central government and the Hong Kong government".
The spokesperson charged unnamed "external forces" with "making all sorts of efforts to complain for and support anti-China and anti-Hong Kong elements."
"Their despicable attempts are disgraceful," they said in a statement.
A European Union spokesperson on Wednesday said the arrests "seem to confirm the EU's concerns about the new law and its effect on the rights and freedoms of the people of Hong Kong".
On Thursday, the United States condemned the arrests "under Hong Kong's repressive new national security law".
"We reiterate our concern that the law's overly broad and vaguely defined provisions appear to further criminalise the exercise of freedom of expression and silence criticism of the government," a US State Department spokesperson said.
Passed by an opposition-free legislature, Article 23 became Hong Kong's second national security law, following a Beijing-imposed security law that came into effect in 2020.
The US, the EU, Japan and Britain have been among Article 23's strongest critics.
China's statement on foreign interference was not connected to the national security trial in which 14 Hong Kong democracy campaigners were found guilty of subversion on Thursday.
