The verdicts bring to an end 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 and Australia 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.
Communist 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 verdicts, 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 later posted on Facebook. AFP has requested confirmation from the police of the arrests.
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!"
Deep concern
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 British Consulate-General also expressed concern, saying in a statement to AFP the case demonstrated "the erosion of meaningful political opposition in Hong Kong".
The United States had already sanctioned six Chinese and Hong Kong officials in response to the 2021 arrests.
China offered no immediate comment on Thursday's decision.
But it released a statement hitting back at Western criticism over the arrest of seven other people this week for social media posts deemed a threat to the government.
"We advise individual countries and politicians to face reality squarely, uphold an objective and impartial stance... and stop interfering in Hong Kong's affairs and China's internal affairs immediately," a foreign ministry spokesperson in Hong Kong said.
Abuse of power
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 and force it to accede to demands raised by protesters in 2019, and ultimately for the city's leader to step down, the court heard.
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.
AFP