HONG KONG, May 28 (Reuters) - Jailed Hong Kong media tycoon and Beijing critic Jimmy Lai is expected to receive a new prison sentence over his role in an unauthorised assembly on Oct. 1, 2019, during one of the city's pro-democracy rallies that year.

This month, Lai, who is serving a 14-month sentence for participating in similar demonstrations on Aug. 18 and Aug. 31, 2019, and nine other activists, pleaded guilty in District Court to organising an unauthorised assembly.

He has been in jail since December after being denied bail in a separate national security trial. He faces three charges under the new law, introduced by China in 2020 in response to the protests, including collusion with a foreign country.

Lai's repeated arrests have drawn criticism from Western governments and international rights groups, who raised concerns over waning freedoms in the global financial hub, including freedom of speech and assembly.

Beijing sees him as a traitor and an anti-China instigator.

China says the sweeping security law, which punishes anything Beijing considers as subversion, secession, terrorism and collusion with foreign forces with up to life in prison, was vital to restore stability and prosperity.

The sentencing is expected to be delivered on Friday by judge Amanda Woodcock, who handed Lai his illegal assembly sentence in April. Other defendants in the trial include activists Lee Cheuk-yan, Figo Chan and Leung Kwok-hung, who is known in Hong Kong as Long Hair.

"Your Honour, for over 40 years I have strived for democratic reform in China," Lee told the court this week during one of the hearings. "This is my unrequited love, the love for my country with such a heavy heart."

It comes two weeks after authorities froze assets belonging to Lai, including bank accounts and his 71.26% stake in media publisher Next Digital.

Hong Kong's security chief sent letters to Lai and branches of HSBC and Citibank this month threatening up to seven years in prison for any dealings with the billionaire's accounts in the city, according to documents seen by Reuters.

The moves could imperil any attempt by the democracy activist to move offshore assets back home to prop up Next Digital's troubled Apple Daily tabloid, a staunch government critic, one of Lai's financial advisers said.