Wen Wei Po and Ta Kung Pao - two Hong Kong newspapers that serve as conduits for Beijing's official policy - also confirmed the passing of the law, as did multiple local Hong Kong media outlets citing anonymous sources in Beijing.
Even as word filtered out that the law had been approved, Hong Kongers remained in the dark about its contents and what might now constitute a crime.
At her weekly press conference on Tuesday morning, Hong Kong leader Carrie Lam - a pro-Beijing appointee - declined to comment on whether the law had been passed or what it contained.
"The fact that Hong Kong people will only come to know what's really in this new law after the fact is more than preposterous," said Claudio Mo, an opposition lawmaker.
Prominent democracy campaigner Joshua Wong tweeted: "It marks the end of Hong Kong that the world knew before. With sweeping powers and ill-defined law, the city will turn into a #secretpolicestate."
Wong and three fellow campaigners announced they were stepping down from Demosisto, the pro-democracy party they founded.
Hong Kong was guaranteed certain freedoms - as well as judicial and legislative autonomy - for 50 years in a deal known as "One Country, Two Systems".
The formula formed the bedrock of the city's transformation into a world class business hub, bolstered by a reliable judiciary and political freedoms unseen on the mainland.
Most brazen move
Critics have long accused Beijing of chipping away at that status in recent years, but they describe the security law as the most brazen move yet.
A summary of the law published by the official state agency Xinhua earlier this month said China's security agencies would be able to set up shop publicly in the semi-autonomous city for the first time.
Beijing has also said it will have jurisdiction over some cases, toppling the legal firewall that has existed between Hong Kong and the mainland's party-controlled courts since the 1997 handover.
Analysts said the security law radically restructures the relationship between Beijing and Hong Kong.
"It's a fundamental change that dramatically undermines both the local and international community's confidence towards Hong Kong's "One Country, Two Systems" model and its status as a robust financial centre," said Hong Kong political analyst Dixon Sing.
On the mainland, national security laws are routinely used to jail critics, especially for the vague offence of "subversion".
Beijing and Hong Kong's government reject those allegations.
They have said that the laws will only target a minority of people, will not harm political freedoms in the city and will restore business confidence after a year of historic pro-democracy protests.