BEIJING: China imposed a sweeping national security law on Hong Kong Tuesday.

As the law was signed by President Xi Jinping little more than six weeks after it was first unveiled, Beijing described it as a "sword" hanging over the heads of those who endanger national security.

The contents of the law have so far been kept secret from Hong Kong's 7.5 million inhabitants, sparking alarm, anger and fear.

"It marks the end of Hong Kong that the world knew before," prominent democracy campaigner Joshua Wong tweeted as his political party Demosisto announced it was disbanding.