The city's weather observatory hoisted the highest-level rainstorm warning for around three hours on Thursday morning, leading the judiciary to cancel hearings for the day.

The 77-year-old founder of the Apple Daily newspaper is charged with foreign collusion under Hong Kong's national security law, which Beijing imposed following huge and sometimes violent pro-democracy protests in 2019.

On Thursday lawyers were expected to present closing arguments as Lai's trial enters its final stages.

The judiciary said Thursday that those due to attend postponed hearings should return on Friday.

The media tycoon has been kept behind bars since December 2020, reportedly in solitary confinement, with Western nations and rights groups calling for his release.

Aside from the collusion offence -- which could land him in prison for life -- Lai is also charged with "seditious publication" related to 161 op-eds he allegedly wrote.

The tycoon gave spirited courtroom testimony over more than 50 days during the trial, fielding questions about his political ideology, management style and overseas contacts.

Lai described himself at least twice as a "political prisoner", which drew sharp rebukes from the three-judge panel.

Hong Kong authorities have repeatedly rejected criticism related to Lai, saying last month that his case was "handled strictly on the basis of evidence and in accordance with the law".

Antoine Bernard of Reporters Without Borders said on Tuesday that Lai's treatment "exposes the authorities' ruthless determination to silence and suppress one of the most prominent advocates for press freedom amid Hong Kong's rapidly deteriorating media landscape".

Judges had previously indicated that a verdict could be reached by October.

Thursday was the fifth time this year that Hong Kong issued the "black" rainstorm warning, beating the previous record of three such warnings in a year.