"Fight on!": Pro-democracy media mogul Jimmy Lai released in Hong Kong

Hong Kong pro-democracy media mogul Jimmy Lai on Wednesday told his reporters to "fight on" following his dramatic arrest, as China widens its crackdown on critics in the semi-autonomous business hub.

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

Lai, 71, was one of ten people arrested by police on Monday as part of an investigation into alleged collusion with foreign forces, a new offence under the security law.

The arrests sparked a new chorus of international criticism that Beijing was stamping out freedoms in Hong Kong, a key regional media hub.

Lai's media group, Next Digital, is unapologetically pro-democracy and critical of Beijing; China hailed his arrest in a series of government statements.

On Wednesday morning Lai was cheered by staff as he toured his Apple Daily newsroom following his late night release on bail after 40 hours in custody.

In images broadcast live on Facebook by his own reporters, he told staff to continue filing the kind of unvarnished dispatches that have infuriated China and pro-Beijing politicians in Hong Kong.

"Fight on! Let's fight on," he said. "We have the support of the Hong Kong people. We can't let them down."

He told staff it was becoming "increasingly difficult" to run a media business in Hong Kong. "But we should continue our work," he added.

"Luckily I was not sent back to the mainland," he quipped in a characteristic display of dark humor.

China has declared it will have jurisdiction over especially serious national security crimes, toppling the legal firewall between the mainland's party-controlled courts and Hong Kong's independent judiciary.