The high-profile national security trial of jailed media tycoon Jimmy Lai, the founder of the now-defunct pro-democracy newspaper Apple Daily, began on Monday, with proceedings expected to last for 80 days.

Jimmy Lai. (Kyodo)

Lai is one of the most prominent figures to be charged under the sweeping national security law, which criminalizes acts of secession, subversion, terrorism and collusion with foreign forces and is punishable by a maximum sentence of life imprisonment.

The 76-year-old has pleaded not guilty to four charges, including two counts of conspiracy to collude with foreign forces and one count of collusion with external forces.

He also faces one count of conspiracy over "seditious publications" linked to Apple Daily, which was forced to end its operations in July 2021 following the freezing of its assets by authorities and the arrest of some of its staff members.

Lai's counsel Robert Pang argued Monday that the prosecution had failed to charge the defendant over the publications within six months of the offence as stipulated by the Crimes Ordinance.

Members of the public began lining up Sunday night near the West Kowloon Magistrates' Courts building to watch proceedings. Over 1,000 police officers will be deployed around the site for the next three months, according to local media reports.

Lai's son, Sebastien Lai, has called numerous times for international help to secure the release of his father, including at a meeting with British Foreign Secretary David Cameron last week.

"I am gravely concerned that anyone is facing prosecution under the National Security Law, and particularly concerned at the politically motivated prosecution of British national Jimmy Lai," Cameron said in an official statement Sunday, calling for Lai's release.

The United States also condemned the prosecution of Lai and sought his immediate release, with State Department spokesman Matthew Miller noting the defendant has been held in pre-trial detention for more than 1,000 days.

Chinese Foreign Ministry spokesman Wang Wenbin called Jimmy Lai "a major mastermind and participant of the anti-China riots" in the former British colony and "an agent and pawn of the anti-China forces" at a press conference in Beijing, adding that "those who break the law must be held accountable."

He said China "firmly rejects" the calls for Jimmy Lai's release by Britain and the United States, saying "making such irresponsible comments on an ongoing judicial process is against the principle of the rule of law."

Beijing firmly supports Hong Kong in "bringing criminals who endanger national security to justice," Wang said. "Attempts to smear, disturb and sabotage Hong Kong's national security law will not succeed," he added.

The trial for the Apple Daily founder was initially slated to begin in December 2022, but was delayed after Hong Kong leader John Lee intervened in the proceedings to bar Lai's defense counsel, veteran British lawyer Timothy Owen, from representing him.