With Friday’s sentence, Lai – who is already behind bars for taking part in earlier protests – must now serve a total of 20 months in jail.
Seven other leading activists, including 25-year-old campaigner Figo Chan, as well as former legislators Lee Cheuk-yan and Leung Kwok-hung, were also given new prison terms of up to 18 months. Two others received suspended sentences.
The new sentences are the latest in a relentless and successful campaign by China to smother dissent in Hong Kong after months of huge and sometimes violent pro-democracy protests in 2019.
The October 1 protest coincided with the 70th anniversary of the founding of communist China and was marked by major clashes, including a live round shot by a policeman at a protester swinging a long stick.
During Friday’s sentencing, Judge Amanda Woodcock noted that those on trial did call for “a peaceful and non-violent procession”.
“But how naive and unrealistic was that considering what was happening on a daily basis was the opposite,” she said. “This is not with hindsight. The risk was very real every day at that time.”
More than 10,000 people were arrested during the protests, with around 2,500 convicted for various offences.
Most of the city’s prominent democracy leaders are now either under arrest, in jail or have fled overseas.
More than 100 people, including Lai, have been charged under the security law, which carries up to life in jail.
The 73-year-old faces three charges under the new legislation, including alleged collusion with a foreign country. He has been in jail since December after being denied bail in the national security trials.
Lai’s repeated arrests have drawn criticism from Western governments and international rights groups, who raised concerns over waning freedoms in the semi-autonomous territory, including freedom of speech and assembly.
Beijing sees him as a traitor and an anti-China instigator.
China says the broadly worded security law, which punishes anything Beijing considers subversion, secession, terrorism and collusion with foreign forces with up to life in prison, is vital to restore stability and prosperity.
Earlier this month, authorities froze assets belonging to Lai, including bank accounts and his 71.26 percent stake in media publisher Next Digital.
Hong Kong’s security chief sent letters to Lai and branches of HSBC and Citibank this month threatening bankers with as many as seven years in prison for any dealings with the billionaire’s accounts in the city, according to documents seen by the Reuters news agency.
The moves could imperil any attempt by the tycoon to bring offshore assets back home and prop up Next Digital’s Apple Daily tabloid, a staunch critic of the government, one of Lai’s financial advisers said.