Lai, a critic of Beijing, is the highest profile person to be arrested under the legislation. He was detained on suspicion of colluding with foreign forces, a crime punishable by up to life in prison.

After initially falling by almost a fifth, shares in Next Digital, which publishes Apple Daily, a pro-democracy, tabloid-style newspaper that also produces investigative work, jumped as much as 344% to their highest since June 2019 at HK$0.40, boosting its market value by HK$817 million ($105.42 million).

Having logged record trading volume, the stock eventually closed up 183%.

A former columnist for the media group, Stanley Wong, said he bought 1.2 million shares before noon to show support for the company and then offloaded them, making a profit of HK$75,000. Wong said he would use the proceeds to fund a scholarship at the Chinese University of Hong Kong.

"The only way I can think of to support Next Media is to buy its stock," Wong said.

Netizens called the surge a "yellow stock circle," referring to companies that support Hong Kong's democracy movement, and "stock with you". One said purchasing shares was the only way to show support and they didn't care if they lost all their money.

Other supporters posted screenshots of their share purchases on Facebook, although Reuters was unable to verify the authenticity of the posts.

Some brokers meanwhile speculated Lai may be forced to sell the group, while market watchers also said the prospect of new leadership at the company could be behind the surge.

"After this event, the market is looking ahead. Maybe, if this chairman is no longer here, another person would take this company in a different direction," said Kenny Ng, an analyst at brokerage Everbright Sun Hung Kai in Hong Kong.

Representatives at Hong Kong markets regulator Securities and Futures Commission and Hong Kong stock exchange declined to comment on the stock's rally.

An Apple Daily executive said the newspaper would be published as usual on Tuesday.

"Even if Apple Daily publish a pile of blank paper tomorrow, we would go and buy a copy of it until it's sold out," democracy activist Joshua Wong said on Facebook, while lawmaker Ted Hui urged people to buy copies of the newspaper no matter what.

($1 = 7.7502 Hong Kong dollars)

(Reporting by Donny Kwok and Clare Jim; additional reporting by Noah Sin and Alun John; Editing by Anne Marie Roantree; Editing by Kirsten Donovan)