Nine pro-democracy campaigners have been convicted over their leadership of “umbrella movement” rallies in Hong Kong in 2014, in a controversial verdict that has prompted renewed alarm about the city’s political freedom.

Protest leaders including the sociology professor Chan Kin-man, 60, the law professor Benny Tai, 54, and the Baptist minister the Rev Chu Yiu-ming, 75, were among those found guilty on rarely used colonial-era public nuisance charges for their roles in the 2014 protests calling for free elections, the largest civil disobedience movement in the city’s history.

All three were charged with one count of conspiracy to cause public nuisance. Tai and Chan were also convicted of inciting others to cause public nuisance. The court did not immediately announce sentences for the charges, which each carry a maximum of seven years in prison.

The other defendants, including the pro-democracy politicians Tanya Chan and Shiu Ka-chun, the former student leaders Eason Chung and Tommy Cheung, and the activist Raphael Wong, were convicted of inciting others to cause a public nuisance as well as inciting others to incite. One former lawmaker, Lee Wing-tat, was found guilty of incitement to incite others to create public nuisance.

In a summary of the judgment, Justice Johnny Chan said civil disobedience was not a defence to a criminal charge. Prosecutors argued that the protests, which shut down parts of the city for almost three months, calling for the rights of Hong Kong residents to elect their own leader, had caused “unreasonable” disruptions to public order.

Riot police fire teargas on student protesters during the 2014 pro-democracy demonstrations. Photograph: Wally Santana/AP

The court said it was “naive to suggest that a concession to introduce the form of universal suffrage ... could be made by the government overnight with a click of the fingers”. The 268-page judgment said it was “equally naive to suggest a mass protest of tens of thousands of people could be dispersed overnight even if a positive response were to come from the authorities”.

On Tuesday, supporters holding yellow umbrellas, a symbol of the demonstrations, crowded outside the courthouse. Tai told supporters: “No matter what happens today, I have the confidence that many people here will strive for Hong Kong’s democracy.”

The trio founded the pro-democracy Occupy Central movement in 2013, which joined the student-led umbrella movement a year later to block reforms that would allow Hong Kong to have direct elections but only of candidates pre-screened by Beijing.

More than two years after the protests ended unsuccessfully, Hong Kong’s justice department brought charges against the group leaders. The nine defendants, known as the “Umbrella Nine”, were the last group of demonstrators to be charged over the 2014 protests.

Human rights groups have described the prosecution as politically motivated, criticising the use of vaguely worded public nuisance laws against protesters.

“Hong Kong courts, by labelling peaceful protests in pursuit of rights as public nuisance, are sending a terrible message that will likely embolden the government to prosecute more peaceful activists, further chilling free expression in Hong Kong,” said Maya Wang, a senior China researcher at Human Rights Watch.

Man-kei Tam, Amnesty International’s Hong Kong director, said: “Today’s guilty verdicts are a crushing blow for freedom of expression and peaceful protest in Hong Kong. The government has used vague charges in their relentless persecution of the Umbrella Nine.

Protesters gather to mark the first anniversary of the start of the Umbrella Movement in 2015. Photograph: Kin Cheung/AP

“The government is increasingly using prosecutions as a political tool to target peaceful activists, abusing the law to silence debate about sensitive issues such as Hong Kong democracy and autonomy.”

The city enjoys rights that do not exist on the Chinese mainland, which are protected by the 50-year handover agreement between Britain and China, but fears are growing that those liberties are being eroded.

The protest movement highlighted widespread frustration, especially among the young, over the direction Hong Kong was going in, but failed to win any reforms or concessions from Beijing. Since then, many activists have been prosecuted, with some jailed, while a string of pro-democracy politicians have been barred.

On Saturday, more than 200 supporters gathered for a service before the verdict where the crowd prayed and sang songs. The venue – Kowloon Union church – was where Tai, Chan and Chu first announced their Occupy Central manifesto six years ago.

“What we are facing is the most powerful autocracy in human history and we have to take back our democratic rights from its hand,” a tearful Tai told his supporters.