Criticism immediately followed the sentencing of dozens of opposition figures including "mastermind" Benny Tai who was given the longest term of 10 years in prison.

A US State Department spokesman called the sentences "unjust", adding that the activists faced "life-altering imprisonment simply for their peaceful participation in political activities."

The United Nations rights office said it was "gravely concerned", while Australia's government said it was advocating for dual citizen Gordon Ng's "best interests" after his sentence of seven years and three months.

The European Union said the jail terms dealt "another unprecedented blow against fundamental freedoms" in Hong Kong.

China responded that Western criticism "seriously desecrates and tramples on the spirit of the rule of law", and warned against interference.

"This case showed that anyone subverting state power and endangering national security will eventually be punished according to law and will not be tolerated," said top city official John Lee, according to the Chinese state-run news agency Xinhua.

All the activists were charged with subversion after holding an informal poll in 2020 as part of a strategy to win a pro-democracy majority in the city's legislature.

The group is made up of some of the most prominent figures of Hong Kong's once-diverse political opposition.

Among them, former student leader Joshua Wong shouted "I love Hong Kong, bye bye!" to the packed courtroom as he was led away after sentencing.

Wong gained international prominence in 2014 as a driving force behind protests known as the Umbrella Movement, featuring on Time magazine's cover when he was just a teenager.

Tai, a constitutional law expert, was given the longest sentence handed out under a national security law brought in to quash dissent after massive, sometimes violent pro-democracy protests in 2019.

Other co-defendants were sentenced to shorter terms beginning from four years and two months.

- 'The wicked shall perish' -

More than 200 people queued in drizzle outside the court since dawn on Tuesday for a public seat.

Inside, the 45 defendants were crammed into the dock, occasionally waving to the public.

They included former lawmakers Claudia Mo and Eddie Chu, and former journalist Gwyneth Ho.

Many have already spent more than 1,300 days in jail.

Outside after the sentencing, the mother of defendant Hendrick Lui silently held up a placard reading: "The righteous shall live, the wicked shall perish."

She was taken away in a police van within seconds, according to video footage from Hong Kong media.

Pro-democracy politicians Au Nok-hin, Andrew Chiu and Ben Chung were singled out as organisers, but received lesser sentences after testifying against Tai.

"Long Hair" Leung Kwok-hung, the 68-year-old co-founder of the city's last standing opposition party, received a term of six years and nine months.

In a letter addressed to his wife and party leader Chan Po-ying, Leung said he had "no regrets."

But he added: "How wonderful it would be if I could say I love you without metal bars between us!"

- Tycoon on trial -

Another closely watched national security trial will see a key development on Wednesday when jailed pro-democracy media tycoon Jimmy Lai testifies in his collusion trial.

Parliamentarians and foreign affairs experts from at least 22 countries and the European Parliament issued an open letter on Tuesday calling for the immediate and unconditional release of Lai, who is a British citizen.

British Prime Minister Keir Starmer raised Lai's case with Chinese President Xi Jinping when they met on the sidelines of the G20 summit in Brazil on Monday.

Those jailed in Hong Kong on Tuesday were accused of subversion after organising a July 2020 primary with the aim of picking a cross-party shortlist of pro-democracy candidates to increase their electoral prospects.

If a majority was achieved, the plan was to force the government to meet their demands -- including universal suffrage -- by threatening to indiscriminately veto the budget.

Three senior judges handpicked by the government to try security cases said that would have caused a "constitutional crisis".

Forty-seven people were initially charged after they were arrested in January 2021.

Thirty-one pleaded guilty, and 16 stood a 118-day trial last year, with 14 convicted and two acquitted in May.