Hong Kong pro-democracy media mogul Jimmy Lai walked free on bail Wednesday, over 40 hours after he and other critics of China were rounded up by police as part of a widening crackdown on dissent.

When Lai left a police station he was swarmed by a crowd of journalists and cheering supporters, some of whom waved copies of his Apple Daily in a show of their backing.

Hong Kongers bought copies of pro-democracy newspaper Apple Daily and shares in its parent company to voice support for its arrested owner Hong Kongers bought copies of pro-democracy newspaper Apple Daily and shares in its parent company to voice support for its arrested owner Photo: AFP / ISAAC LAWRENCE

A clampdown has gathered pace in Hong Kong since China imposed a sweeping security law in June, with opposition politicians disqualified and activists arrested for social media posts.

The moves have provoked outrage in the West and fear for millions who last year took to the streets to protest communist China's tightening grip on the semi-autonomous city.

Prominent Hong Kong democracy activist Agnes Chow is arrested under a new national security law
"It's now confirmed that Agnes Chow has been arrested for 'inciting secession' under the national security law," her Facebook account said. A police source c Prominent Hong Kong democracy activist Agnes Chow is arrested under a new national security law "It's now confirmed that Agnes Chow has been arrested for 'inciting secession' under the national security law," her Facebook account said. A police source confirmed to AFP that Chow was among 10 people arrested on Monday in a national security investigation. Photo: AFPTV / Daniel SUEN

In one of the most dramatic days of the crackdown, Lai was among 10 people detained under the new law on Monday as around 200 police officers searched the newsroom of his tabloid, which is unapologetically critical of Beijing.

Lai did not address the crowd upon his release, but he flashed a thumbs up as he was bundled into a car that inched away through the crowd.

Jimmy Lai had long expected his pro-democracy views would see him arrested Jimmy Lai had long expected his pro-democracy views would see him arrested Photo: AFP / VERNON YUEN

In a display of solidarity for Lai, people in the city rushed to buy Tuesday's Apple Daily, with the newspaper saying it had upped its print run to 550,000 from the normal circulation of 70,000.

One restaurant owner bought 50 copies at a newsstand in the commercial district of Mong Kok and said he planned to give them away free of charge.

Thousands of people in Hong Kong have been arrested during more than a year of pro-democracy protests Thousands of people in Hong Kong have been arrested during more than a year of pro-democracy protests Photo: AFP / DALE DE LA REY

"Since the government doesn't allow Apple Daily to survive, then we as Hong Kongers have to save it ourselves," the man, who gave his surname as Ng, told AFP, as dozens of people lined up around the city from the early hours.

The newspaper's front page showed a picture of Lai being led away in handcuffs, with the headline "Apple will fight on".

Protests last year often turned violent, with police firing tear gas and rubber bullets Protests last year often turned violent, with police firing tear gas and rubber bullets Photo: AFP / DALE DE LA REY

Lai's arrest sparked a buying spree in shares of his media group, and between Monday morning and closing time on Tuesday its stock value had risen by more than 1,100 percent.

Jimmy Lai had long expected his pro-democracy views would see him arrested Jimmy Lai had long expected his pro-democracy views would see him arrested Photo: AFP / VERNON YUEN

Hong Kong's new national security law criminalises secession, subversion, terrorism and colluding with foreign forces.

The most serious crimes under the law -- which was introduced on June 30 and is not supposed to be retroactive -- carry up to life in jail.

Chart showing share price for Apple Daily parent company Next Digital Ltd. Chart showing share price for Apple Daily parent company Next Digital Ltd. Photo: AFP / Laurence CHU

Its broadly worded provisions criminalised certain political speech overnight, such as advocating sanctions, and greater autonomy or independence for Hong Kong.

Similar laws are used on the authoritarian mainland to snuff out opposition.

Lai, 71, was held on charges including colluding with foreign forces and fraud. The operation was hailed by Beijing, quick to declare him an "anti-China rabble-rouser" who conspired with foreigners to "stir up chaos".

Among the others arrested were two of Lai's sons, young pro-democracy activist Agnes Chow and Wilson Li, a former activist who works as a freelancer for Britain's ITV News.

Chow was released on bail late Tuesday.

"It's very obvious that the regime and the government are using the national security law to suppress political dissidents," she told reporters after her release.

Critics believe the security law has ended the key liberties and autonomy that Beijing promised Hong Kong could keep after its 1997 handover by Britain.

US Secretary of State Mike Pompeo described Lai's arrest as "further proof" that Chinese authorities had "eviscerated Hong Kong's freedoms and eroded the rights of its people".

"We're going to respond in real ways," Pompeo later promised in an interview with Newsmax.

The United States has already imposed sanctions on a group of Chinese and Hong Kong officials -- including city leader Carrie Lam -- in response to the crackdown.

Hong Kong's police said those arrested were part of a group that had previously lobbied for foreign sanctions.

"We call on the FCC, Hong Kong to respect the facts, distinguish right from wrong, and stop smearing under the pretext of press freedom the implementation of the National Security Law," it said.