HONG KONG -- Jimmy Lai had just finished his morning exercise and was catching up on the news when the police arrived.

"There were 15, 20 policemen there, and they said I was under arrest and that they were from the national security department," the Hong Kong media tycoon recounted in a live-streamed video chat Thursday.

His arrest Monday morning at his home was the highest-profile detention under a new national security law that took effect in the semi-autonomous Chinese territory on June 30. He has not been charged but was held for 36 hours on suspicion of colluding with a foreign country or entity. He was released on bail.

Lai, 71, said he was worried he could be sent to the mainland, where the legal system has fewer protections. His fears were allayed when he realized that the officers were not speaking in Mandarin, the mainland Chinese language.

"I kind of settled down a little bit because ... knew that I wouldn't be sent to China at least," he said.

The national security law says that certain cases can come under the jurisdiction of mainland China. Chinese and Hong Kong officials say most cases will be handled locally.

In the hours after his arrest, Lai was taken to his office and his personal yacht as police searched for evidence. He was also shifted between several police stations, as there was an electricity blackout in one, while in another the fingerprint machine was not working.

"A lot of the time I was sitting there waiting," he said, adding it was the first time he had spent so much time in handcuffs as police took him from place to place.

The headquarters of his Next Digital media group, which operates the pro-democracy Apple Daily tabloid, was raided by hundreds of officers after his arrest, stoking fears that press freedom would be curtailed.

Hong Kong has long enjoyed freedoms not found on the mainland under a "one country, two systems" framework.

Lai expressed surprise that his arrest came so quickly, saying that he thought authorities would keep a low profile because of the strong condemnation of the law by some in the international community.

He has been a harsh critic of the new law and China's ruling Communist Party.

"If I knew that I will end up here or eventually in prison, would I have changed myself in other way? I thought about it, and I said no, because this is my character," he said. "This is the way I react to things. My character is my destiny."

Following Lai's arrest on Monday, investors rushed to buy Next Digital stock in a show of support. The stock surged more than 1,100% to 1.61 Hong Kong dollars before settling around 0.38 Hong Kong dollars on Thursday. That's still nearly five times higher than before Lai's arrest.

He urged people not to engage in a buying spree of Next Digital shares.

"I advise nobody to touch it," he said. "Because this is just a very ephemeral phenomenon that's not going to last."