HONG KONG, Feb 1 — Detained Hong Kong media tycoon Jimmy Lai was in court today trying to win bail in the first major legal challenge to a sweeping national security law Beijing imposed on the city last year.

Lai, 73, who owns the pro-democracy tabloid Apple Daily, is one of more than 100 democracy supporters arrested under the new law since it was enacted in June and he is the highest profile figure to be incarcerated on pretrial charges.

He has been charged with "colluding with foreign forces” — one of the new national security crimes — for allegedly calling for sanctions on Hong Kong and China in response to an ongoing crackdown on pro-democracy activism.

The security law is the most dramatic shift in Hong Kong’s relationship with authoritarian China since the business hub was handed back by Britain in 1997, criminalising a host of political views and toppling the legal firewall between the two territories.

Written in Beijing and imposed by fiat, it allows mainland security agents to operate openly in the city for the first time and grants China jurisdiction in some cases.

Another precedent-setting element of the law is no presumption of bail — a hallmark of Hong Kong’s independent common law legal system.

That is what Lai and his lawyers were seeking to challenge today. 

Lai was detained in December and initially granted bail after he agreed to a stringent list of requirements, including house arrest and no interviews or social media posting.

But he was put back behind bars days later by the Court of Final Appeal pending this week’s full hearing.

A panel of top judges will now have to balance the wording of Beijing’s law against the city’s common law traditions, its mini-constitution and its Bill of Rights which supposedly guarantee freedom of speech and a presumption of bail for non-violent crimes.

Legal analysts say the outcome will give an indication of whether Hong Kong’s judiciary will serve — or even can serve — as any kind of constitutional brake against Beijing’s security law.

"Today’s hearing... is likely to be the single most important and consequential court hearing in Hong Kong’s post-1997 history,” Antony Dapiran, a Hong Kong lawyer and author, told AFP.

"It will be the first opportunity Hong Kong’s highest court has to express its views on the National Security Law and how it can be expected to dovetail with Hong Kong’s common law system and existing protections of human rights,” he added.

The judges are in a precarious position. 

In Hong Kong’s complex constitutional hierarchy the ultimate arbiter of the laws is Beijing’s Standing Committee which has showed an increased willingness in recent years to wade into legal arguments and make pronouncements.

China’s state media have already declared Lai guilty and made clear authorities expect Hong Kong’s judges to side with Beijing on national security, meaning any move to the contrary could put the judiciary on a collision course with the central government. — AFP