A Hong Kong court found 14 people guilty of subversion on Thursday in the biggest case against pro-democracy campaigners since China imposed a national security law to crush dissent.

The verdicts cap a long-running trial in which 47 people were charged for organising an unofficial election in 2020, activities the court ruled were a threat to the government.

Fourteen people were found guilty on Thursday. They, along with 31 others who pleaded guilty, could face life in jail. Two were found not guilty.

"In our view... that would create a constitutional crisis for Hong Kong," said a court statement summarising the reasons for the verdict by three judges.

Britain, Australia and the European Union quickly condemned the verdicts with language that echoed the frequent criticism from Western governments in recent years over China's crackdown on democracy in the former British colony.

China imposed a national security law on Hong Kong in 2020 after millions of people took to the streets in sometimes violent pro-democracy protests.

The 47 were arrested in dawn raids in January 2021 and charged with subversion for organising the election, which aimed to shortlist candidates for the city's parliament.

The arrests were part of a campaign to eliminate dissent that in recent years has transformed Hong Kong, which Beijing had promised to allow freedoms when it took the territory back from Britain in 1997.

Lawrence Lau, one of the defendants found not guilty, called for people to keep supporting the rest of the group.

"I hope that everyone will continue to (have) concern for our other friends in the case," he told reporters outside court.

Hong Kong authorities announced later they would appeal the two not guilty verdicts.

Ahead of the announcements, a small group from the League of Social Democrats -- one of the few remaining opposition voices in Hong Kong -- attempted to stage a small protest.

"Hong Kong should still be a place with freedom of expression and of assembly," said chairperson Chan Po-ying, who is also the wife of defendant "Long Hair" Leung Kwok-hung.

Chan and three others were later arrested, activist Figo Chan posted on Facebook. The police confirmed the campaigners' arrest for "actions that disrupted public order".

Well-known activist Alexandra Wong, known as Grandma Wong, also attempted to stage a protest before police moved her across the street to a fenced-off area.

"Immediately release the 47!" she shouted, waving a British flag. "Support democracy, support the 47!"

Britain on Thursday said the guilty verdicts showed how Hong Kong has used the Beijing-imposed national security law to "stifle opposition and criminalise political dissent".

The convicted campaigners were "guilty of nothing more than seeking to exercise their right to freedom of speech, of assembly and of political participation", junior foreign minister Anne-Marie Trevelyan said in a statement.

The European Union said the convictions mark "a further deterioration of fundamental freedoms and democratic participation in Hong Kong".

Australian Foreign Minister Penny Wong voiced deep concern over the convictions, adding she would raise the fate of an Australian citizen among those found guilty at the "highest levels".

The United States had already sanctioned six Chinese and Hong Kong officials in response to the 2021 arrests.

China said it "firmly supports" Hong Kong's law enforcement and judicial authorities in "punishing" acts that violated national security.

"No one can engage in unlawful activities in the name of democracy and then try to escape the sanction of the law," foreign ministry spokeswoman Mao Ning told a regular briefing.

"We resolutely oppose any interference in China's internal affairs by individual countries."

Prosecutors said the 47 had conspired to subvert state power by holding the unofficial primary polls, which aimed to secure a parliamentary majority for the pro-democracy bloc.

They would then veto government budgets, force it to accede to demands raised by protesters in 2019 and ultimately push for the city's leader to step down, prosecutors said.

Defence lawyers argued Hong Kong's mini-constitution allowed for such manoeuvring and that the matter was "a purely political issue rather than a legal matter".

But the court ruled the actions were an "abuse of power".

Before Thursday, 114 people had been found guilty of crimes related to the national security law since it was introduced.

The case against the group of 47 was the biggest under the law.

Their trial was held without a jury and the judges were chosen from a pool of jurists handpicked by Hong Kong's leader.