One of the activists said they could still be charged under a tough national security law.
The activists were accused of taking part in an unofficial primary election last year that authorities said was part of a plan to paralyse the Legislative Council and subvert state power.
The primary was held to choose the best candidates to field as the pro-democracy camp sought to win a majority of seats.
The mass arrests were the biggest move against Hong Kong’s democracy movement since Beijing imposed the national security law in the semi-autonomous territory last June to quell dissent following months of anti-government protests in 2019.
Three of the 55 arrested people were not released, activists Joshua Wong and Tam Tak-chi, who were already in jail on separate charges, and former Hong Kong Democratic Party chairman Wu Chi-wai, who remained in custody for failing to meet bail conditions in a separate protest-related case.
Mr Wu had been granted bail last month after being charged with inciting others to join unauthorised protests in July 2019.
He was ordered to surrender his travel documents, and authorities later learned that he did not hand in his British National Overseas passport.
Police said none of the activists arrested Wednesday on suspicion of subversion under the security law had been formally charged.
Former pro-democracy politician Lam Cheuk-ting told reporters that determinations of whether to formally prosecute people in Hong Kong are not based on evidence, but on a “political decision” by officials.
“They haven’t made any charges against us yet,” said Mr Lam, who was among those arrested on Wednesday.
“But I’m quite sure that they will charge some of us sooner or later, whether they have sufficient evidence or not.”
He said the purpose of the arrests was to silence Hong Kong’s people and create a “chilling effect.”
Separately, three people were sentenced to up to five and a half years in prison on Friday for rioting at Hong Kong’s airport and assaulting a Chinese journalist in August 2019 at the height of the anti-government protests.
Protesters blockaded the city’s airport for two days.
Some turned on individuals they believed were spies or undercover police, and a reporter from China’s state-owned Global Times newspaper was tied up and attacked by a group of protesters.
Al-Arabiya TV: Saudi minister optimistic US ties will be ‘excellent’ under Biden
Appointments made by Biden show “understanding of the common issues” by the new US administration, Prince Faisal bin Farhan added, according to Twitter posts by the Saudi-owned, Dubai-based TV channel.
“The Biden administration will find that our targets regarding Yemen are the same,” Al-Arabiya quoted him as saying.
READ: Biden administration to declassify report about Khashoggi murder
A Saudi-led military coalition intervened in Yemen in 2015, backing government forces fighting the Iran-aligned Houthi group after it seized the capital Sanaa.
Biden pledged during his election campaign to reassess ties with Saudi Arabia, demanding more accountability over the killing of Saudi journalist Jamal Khashoggi in 2018 and calling for an end to US support for the Saudi-led campaign in Yemen.
Saudi Arabia and other Gulf Arab monarchies have long been allies of the United States, sharing the objective of protecting oil supplies from the Gulf region and containing Iran.
Too early to say when lockdown will end says Johnson
Britain posted a fresh record in daily deaths on Wednesday for the second day running, hitting 1,820, figures that Mr Johnson has called “appalling”. The daily death count dropped on Thursday.
A prevalence survey, known as REACT-1, suggested infections had not fallen in the first days of lockdown, though the UK government has said that the impact of national restrictions introduced on January 5th was not yet reflected in the numbers.
“I think it’s too early to say when we’ll be able to lift some of the restrictions,” Mr Johnson told broadcasters.
“What we’re seeing in the ONS data, in the REACT survey, we’re seeing the contagiousness of the new variant that we saw arrive just before Christmas. There’s no doubt it does spread very fast indeed.”
England’s third national lockdown has seen bars, restaurants and schools mostly closed, with Mr Johnson attributing a steep rise in cases at the end of last year to a more transmissible variant of the coronavirus first detected in England.
While deaths have been rising, the number of new cases has fallen from a peak of 68,000 on January 8th to 37,892 on Thursday.
Johnson aims to vaccinate high priority groups by mid-February to protect the eldest, the most vulnerable and frontline health staff, before considering whether restrictions can be eased.
The UK delivered 363,508 vaccines on Wednesday, a new daily record that took the number of people to have received a first dose to 4.97 million.
Ministers have appealed to people to stay at home as much as they can to prevent hospitals being overwhelmed, and they increased the fines people will pay if they break the rules.
Vin Diwakar, the regional medical director for London, likened anyone breaking the rules to someone who turned the lights on during a blackout in World War Two.
“It doesn’t just put you at risk in your house, it puts your whole street and the whole of your community at risk,” he said.
Meanwhile, the government’s top scientific adviser described some hospitals as looking like a war zone.
Joe Harrison, chief executive of Milton Keynes University Hospital NHS Foundation Trust, said the hospital had seen more than twice the number of patients in the second wave than the first, and currently had 186 patients with Covid-19.
“We believe that over the next week or so, we’re going to continue to see real pressures in our critical care unit,” he told Reuters. “And then hopefully we will turn the corner and things will start to improve.”