Alexandra Wong, an activist known as Grandma Wong, walks with a British Union Jack flag outside West Kowloon court in Hong Kong on Thursday during court appearances by dozens of dissidents charged with subversion in the largest use yet of Beijing's sweeping new national security law. Photo: AFP / Anthony Wallace

Dozens of Hong Kong dissidents charged with subversion were sent to jail on Thursday after a judge denied bail at the end of a marathon hearing that has reignited international alarm over China’s crackdown on dissent in the finance hub.

Police charged 47 of the Chinese city’s best-known dissidents on Sunday for “conspiracy to commit subversion” in the broadest use yet of a sweeping national security law that Beijing imposed on the city last year.

The defendants represent a wide cross-section of Hong Kong’s opposition, from veteran former pro-democracy lawmakers to academics, lawyers, social workers and youth activists.

What followed was an extraordinary four day bail hearing that starkly illustrated the profound changes Beijing’s security law has begun making to the financial hub’s legal traditions.

On Thursday evening, after hours of legal arguments from prosecutors and defence, chief magistrate Victor So ordered 32 members of the group to be returned to custody. 

“The court does not find it has sufficient ground to believe that you will not continue to commit acts that endanger national security,” he told the court.

The remaining 15 defendants were granted bail with strict limitations but the prosecution immediately appealed. 

As a result those 15 must also return to custody but can take their case to the city’s High Court within the next 48 hours following the decision.