The verdicts, as well as two acquittals, in Hong Kong's biggest trial against the democratic opposition come more than three years after police arrested 47 democrats in dawn raids at homes across the city.
They were charged with conspiracy to commit subversion under a national security law imposed by China.
Sentencing will come at a later date for those found guilty on Thursday, with prison terms ranging from three years to life for this offence.
Thirty-one defendants pleaded guilty, and four of them have become prosecution witnesses.
Among the 14 convicted were former Hong Kong district councillor Michael Pang. (AP PHOTO)
The US and some other countries have criticised the trial as politically motivated, calling for the accused to be immediately released.
Diplomats from several countries including the US and the European Union attended the hearing.
Australian Foreign Minister Penny Wong said her government was "deeply concerned" with the verdicts and would continue to seek consular access to Gordon Ng, an Australian citizen and one of those convicted.
Security was tight around the High Court, with scores of police officers, some with dogs, and vehicles patrolling the area. Some supporters queued overnight to secure a spot.
The defendants are accused of a "vicious plot" to paralyse government in the former British colony and force the city's leader to resign through a preselection ballot in a July 2020 citywide election.
The democrats maintain it was an unofficial attempt to select the strongest candidates in a bid to win a historic majority in Hong Kong's legislature.
Judges Andrew Chan, Alex Lee and Johnny Chan wrote that if the defendants had succeeded, it would have created "a constitutional crisis for Hong Kong" and led to "serious interfering in, disrupting or undermining the performance of duties and functions in accordance with the law by the (Hong Kong) government".
Several defendants, including activists Owen Chow, 27, and Gwyneth Ho, 33, appeared stony-faced in the dock as the verdicts were delivered to a packed courtroom.
Barrister Lawrence Lau was one of two defendants acquitted by the court. (AP PHOTO)
Those convicted also include former democratic MPs Leung Kwok-hung, Lam Cheuk-ting and Helena Wong, and former district councillor Michael Pang.
It was not yet clear whether any would appeal.
Leung, 68, is the oldest defendant.
"Although he might not be well emotionally and still not yet adapted to prison life... he always told me that he's innocent," Leung's wife, Chan Po-ying, told Reuters.
Acquitted were barrister Lawrence Lau and social worker Lee Yue-shun.
"There are still other defendants in this case warranting our concern and even love," Lau said outside the court.
The prosecution in the afternoon applied to appeal the two acquittals.
Mass pro-democracy protests erupted in Hong Kong in 2019 against Beijing's plans for legislation that democrats argued infringed on freedoms guaranteed when Hong Kong returned to China's control in 1997.
Mass pro-democracy protests that erupted in 2019 led to a sweeping national security law. (AP PHOTO)
Beijing in 2020 imposed the sweeping national security law that led to a spate of arrests of democratic campaigners as well as the closures of liberal media outlets and NGOs.
Hong Kong's democratic opposition had sought for decades to pressure Beijing to allow full democracy in the city.
Once-lively street marches, demonstrations and vigils have essentially ceased amid intense policing.
"This unprecedented mass conviction is the most ruthless illustration yet of how Hong Kong's national security law is weaponised to silence dissent," Amnesty International's China director, Sarah Brooks, said in a statement.
"It represents a near-total purge of the political opposition."
Beijing says the national security laws have brought stability to Hong Kong and that human rights are respected.
Most of the accused have been detained since February 28, 2021, and went through marathon bail hearings.
Those who have pleaded guilty include former law scholar Benny Tai, whom the prosecution called a "mastermind" of the "conspiracy", and activist Joshua Wong.
