Hong Kong police moved in swiftly on protesters, Wednesday (July 1), and made their first arrests under Beijing's sweeping new security legislation, for holding a Hong Kong independence flag.
The crowd continued to swell in the thousands, chanting liberate Hong Kong.
Water cannon was fired, and riot police used pepper spray on protesters and press at close range.
More than 70 arrests were made for illegal assembly, obstruction, possession of weapons, and violating the new law, which was marked be a purple flag held by the police, citing the new rules for the first time.
Beijing unveiled the details of the much-anticipated law on Tuesday (June 30) after weeks of uncertainty.
The law punishes crimes of secession, subversion, terrorism and collusion with foreign forces - with up to life in prison, heralding a more authoritarian era for China's freest city - and the Asian financial hub.
Authorities in Beijing and Hong Kong have repeatedly said the legislation is aimed at a few "troublemakers" and will not affect rights and freedoms.
The protests followed a flag-raising ceremony Wednesday morning to mark the 23rd anniversary of the former British colony's handover to China.
Leader Carrie Lam said the law was the most important development since the city's return to Chinese rule.
"The enactment of the national law is regarded as the most significant development in the relationship between the Central Authorities and the HKSAR since Hong Kong's return to the Motherland. It is a historic step to improve the system in Hong Kong to safeguard our country's sovereignty, territorial integrity and security."
The new law will supersede existing Hong Kong laws where there is a conflict - and interpretation powers of the law belong to Beijing.
Judges for security cases will be appointed by the city's leader.
The law has been widely condemned around the globe.
And Taiwan said on Wednesday they had opened an office to help people fleeing from Hong Kong.