They urged China to reconsider, saying the law "undermines" the city's freedoms.
It remains unclear whether Hongkongers will heed the call to protest given the risks posed by the new security law - which came into effect overnight - and increasingly aggressive police tactics towards even peaceful gatherings in recent months. About a dozen participants chanted slogans echoing demands from protesters past year for political reform and an investigation into accusation of police abuse.
Rights and freedoms, including freedom of speech, of the press, of publication, of assembly and demonstration, will be protected in accordance with the law. The law even has restrictions on all types of Non violent protest.
The controversial legislation outlaws activities pertaining to separatism, subversion, terrorism and collusion in support of foreign interference.
He said the new powers, which could restrict protests and freedom of speech, threatened the city's autonomy.
"What's this got to do with you?" said Zhang Xiaoming of the Hong Kong and Macao Affairs office of the State Council.
China's parliament adopted it in response to months of pro-democracy protests past year triggered by fears that Beijing was stifling the city's freedoms, guaranteed by a "one country, two systems" formula agreed when it returned to Chinese rule.
Recently, a high-level Chinese military officer was arrested and charged with J1 visa fraud, after being caught stealing research from the University of California.
"As Beijing moves forward with passing the national security law, the United States will today end exports of USA -origin defense equipment and will take steps toward imposing the same restrictions on US defense and dual-use technologies to Hong Kong as it does for China", Pompeo said in a statement.
Hong Kong's chief executive Carrie Lam said the new law would fill gaps in the territory's national security legislation and would only affect a small number of people.
The law that has 66 articles in six chapters clearly defines the duties and government bodies of the HKSAR for safeguarding national security; the offenses and their corresponding penalties; jurisdiction, applicable law and procedure; office of the central people's government for safeguarding national security in the HKSAR; and other contents.
"Refugees from Hong Kong will not be subjected to the numerical limitation", a release states. Having lost patience, Beijing finally chose to circumvent the Hong Kong legislature and have it passed Tuesday by the Standing Committee of the National People's Congress, China's rubber-stamp parliament.
There is concern in Hong Kong that the prohibition of "terrorist activity" will be applied broadly and arbitrarily.
On Wednesday morning, Hong Kong politicians and dignitaries gathered in far greater numbers than those legally allowed by the city's anti-pandemic measures for a flag-raising ceremony to mark the anniversary.
The end of Hong Kong self-governance?
The U.S.is moving to end special trade terms given to the territory.
The Trump administration has also said it will bar defense exports to Hong Kong and will soon require licenses for the sale of items that have both civilian and military uses.
Congress has also moved to impose sanctions on people deemed connected to political repression in Hong Kong, including police officials, while Britain has said it could offer residency and possible citizenship to about 3 million of Hong Kong's 7.5 million people.