He pleaded not guilty to all charges. Sentencing is expected to come at a later date.
Under Hong Kong's sweeping national security law, Lai could face up to life imprisonment.
The case has drawn international scrutiny of Hong Kong's judicial independence amid a years-long crackdown on rights and freedoms in the global financial hub.
Lai, 78, was arrested in August 2020 under a Beijing-imposed national security law that was implemented following massive anti-government protests in 2019.
During his five years in custody, Lai has been sentenced for several lesser offences, and appears to have grown more frail and thinner.
After entering the courtroom on Monday, Lai smiled and waved to the public gallery. Among the attendees were Lai's wife and son, and Hong Kong's Roman Catholic Cardinal Joseph Zen.
Lai's trial, heard by three judges approved by the government without a jury present, has been closely monitored by the US, Britain, the European Union and political observers as a barometer of media freedom and judicial independence in the former British colony, which returned to Chinese rule in 1997.
with AP
