China passed a sweeping national security law for Hong Kong on Tuesday, a historic move that critics and many western governments fear will smother the finance hub's freedoms and hollow out its autonomy.

The legislation was unanimously approved by China's rubber-stamp parliament, little more than six weeks after it was first unveiled.

"It marks the end of Hong Kong that the world knew before," prominent democracy campaigner Joshua Wong tweeted. "With sweeping powers and ill-defined law, the city will turn into a #secretpolicestate."

The United States, Britain, the European Union and the United Nations rights watchdog have all voiced fears the law could be used to stifle criticism of Beijing, which wields similar laws on the authoritarian mainland to crush dissent.

The law bypassed Hong Kong's fractious legislature and the wording was kept secret from the city's 7.5 million inhabitants.

The opacity continued even after the law was passed, with silence from Beijing. Instead the news filtered out via pro-Beijing politicians and local media outlets in Hong Kong.

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," Claudia Mo, an opposition lawmaker, told AFP.