Hong Kong media mogul Jimmy Lai, who was arrested Monday under the nation’s new national security law, urged young protesters to be “more cautious” in the fight ahead.

Lai, who owns the Apply Daily, one of Hong Kong’s most-read newspapers, is one of the most prominent figures to be detained under a wider crackdown on political dissidents.

FILE: Hong Kong media tycoon Jimmy Lai pauses during an interview in Hong Kong.  (AP)

Nine other people – including two of Lai’s sons – were arrested Monday on charges of foreign collusion, under the national security law that China implemented in June, the Wall Street Journal reported.

Lai was released on bail early Wednesday. Speaking to the BBC after his release, he said his arrest was “just the beginning.”

"When I was in custody I could not sleep,” Lai said. “I was thinking, if I knew that was going to happen to me now, [with] even more hardship [on the way], would I have done the same thing?

Lai said he has no regrets in his actions but warned young protesters to be “more cautious in our resistance to preserve our rule of law and freedom.”

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"We have to be more careful and creative in [our] resistance... we can't be as radical as before -- especially young people -- because the more radical [we are] the shorter lifespan we have in our fighting,” Lai said. "We have to really use our brain and patience, because this is a long fight."

Lai’s arrest has galvanized pro-democracy activists. On Tuesday, scores lined up at newsstands across the city to buy that same paper, handing over 10 Hong Kong dollars ($1.25) a copy in a bid to help the Apple Daily — and press freedom — survive.

People queue up at a news stand to buy copies of Apple Daily at a downtown street in Hong Kong Tuesday, Aug. 11, 2020, as a show of support, a day after the arrest of its founder Jimmy Lai.  (AP)

The mass arrests have stoked fears that the new national security law would be used to suppress dissent in Hong Kong after months of anti-government protests shook the city's leadership and the central government in Beijing last year.

Police have expanded their use of the law since it took effect six weeks ago, first arresting protesters with slogans deemed to be in violation and then activists over online posts.

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The Apple Daily, known for its celebrity coverage as well as its condemnation of China's authoritarian rule, remained defiant, printing 350,000 copies — five times its usual print run — after police investigators left Next Digital and told employees they could go back to work.

The Associated Press contributed to this report.