Among those detained in the swoop were prominent Democratic Party founder and senior barrister Martin Lee, 81, millionaire publishing tycoon Jimmy Lai, 71, and former legislator and barrister Margaret Ng, 72, according to media and political sources.
In all, nine former legislators were arrested.
Democratic legislator Claudia Mo, who was not among those arrested, said the city government, led by Chief Executive Carrie Lam, was trying "into introduce a ring of terror in Hong Kong".
"They are doing whatever they can to try to silence, to take down, the local opposition," Mo said, pointing to upcoming legislative elections in September in which democrats hope to win back veto power in the city assembly.
Hong Kong police superintendent Lam Wing-ho told reporters that 14 people aged between 24 and 81 were arrested on charges of organising and participating in "unlawful assemblies" on Aug. 18 and Oct. 1 and 20 last year. He did not identify the 14.
Those days saw big and at times violent protests across the city.
Five of the 14 were also arrested for publicising unauthorised public meetings on Sept. 30 and Oct. 19, Lam said.
They were all due to appear in court on May 18, but Lam said more arrests were possible. It is not known whether any of those arrested on Saturday were being held in detention.
The raids mark the biggest crackdown on the pro-democracy movement since the beginning of the anti-government protests across the former British colony in June last year.
NEW PUSH FOR SECURITY LAW
Marchers initially targeted a now-scrapped bill proposing to send suspects to mainland China for trial but protests broadened into demands for full democracy and a public investigation of the use of force by police.
Lai was arrested on similar charges in late February, along with veteran activists Lee Cheuk-yan and Yeung Sum, who were also arrested on Saturday.
After his release on Saturday afternoon, Martin Lee said he felt "relieved to be listed as a defendant because I have seen many brilliant young people being arrested but I didn't."
"I don't regret what I have done," he added. "I'm proud to have the chance to walk our democracy road with Hong Kong's excellent young people."
A spokesman for the government's Security Bureau said that regardless of background or status, "in Hong Kong, everyone is equal before the law".
"No-one is above it nor can anyone break it without facing consequences … The police will handle the case in a fair, just and impartial manner," the spokesman said.
The arrests come after several months of relative calm amid a partial coronavirus lockdown but just as Chinese and city government officials launch a new push for tougher national security laws for Hong Kong.
The Asian financial hub returned to Beijing in 1997 under a "one country, two systems" formula that guarantees it broad freedoms not seen in mainland China, and a high degree of autonomy.
A previous attempt to draft a national security law for Hong Kong, known as Article 23, was met with mass protests in 2003 and abandoned.
Hong Kong government and security officials have recently described some of the democracy movement's actions as being close to terrorism.
Authorities are increasingly using the threat of terrorism to justify the need for new national security laws, a requirement under the Basic Law – the mini-constitution that guarantees Hong Kong's broad freedoms and outlines its relationship with Beijing.
Authorities in Hong Kong have arrested more than 7,800 people over their involvement in the last year's protests, including many on rioting charges that can carry jail terms of up to 10 years.
It is not clear how many of them are in custody. China's state media has repeatedly demanded Hong Kong's independent judiciary take tough measures against protests.