Shares in the parent company of Hong Kong's pro-democracy Apple Daily newspaper quadrupled on Thursday as trading in them resumed following an eight-day suspension.

Dealing in Next Digital was halted last week after city authorities said they were freezing the assets of its jailed owner Jimmy Lai, using a sweeping new national security law.

But when trading resumed Thursday the share price skyrocketed from its previous close of HK$0.19 to hit HK$0.80 before it closed at HK$0.28 -- 51 percent up.

The group saw a similar -- and volatile -- surge in its share price in August when police first arrested Lai, as ordinary Hong Kongers snapped up the firm to show support for the embattled Apple Daily.

The 73-year-old media tycoon has long been a thorn in Beijing's side for his caustic newspaper and unapologetic support for democracy.

Apple Daily, the city's most popular tabloid, has staunchly backed Hong Kong's pro-democracy cause, including the huge and often violent protests that swept the international financial hub in 2019.

In a filing on Wednesday announcing its plan to start trading again the following day, Next Digital said HK$500 million ($65 million) of Lai's assets had been frozen, including his 71 percent stake in the company.

The freeze did not extend to Next Digital itself or its subsidiaries, the company said.

It was the first time Hong Kong authorities have used the national security law to freeze shares of a listed company's majority shareholder -- a step that could cause further jitters for investor sentiment in the city.

Apple Daily is Hong Kong's most popular tabloid and a staunch supporter of the city's pro-democracy movement
Apple Daily is Hong Kong's most popular tabloid and a staunch supporter of the city's pro-democracy movement AFP / Peter PARKS

On Thursday, Hong Kong's security chief confirmed he had sent written notices to two banks saying that any dealings with the tycoon's frozen accounts would be subjected to fines and jail terms.

"It is my duty to specify in my notice to the relevant parties, what will be the consequences if they fail to comply with my direction," John Lee told media.

He added that the letters were targeted at properties related to national security offences and that "normal businessmen will go about their duties and have nothing to do with endangering national security".

Next Digital is in a precarious position. Its unaudited bank and cash balances were approximately HK$521.4 million at the end of March.

"Existing working capital of the group will remain sufficient for at least 18 months from April 1, 2021 without additional funding from Mr Lai," the filing said.

The asset freeze comes as Chinese authorities roll out a relentless and successful campaign to silence dissent in Hong Kong.

Lai is currently serving a 14-month jail sentence for taking part in two of the 2019 protests.

He is also charged with "colluding with foreign forces" -- one of the new national security crimes -- for allegedly advocating for foreign sanctions.

More than 100 people -- including some of the city's most prominent democracy activists -- have been arrested under the law.

Most of those charged are refused bail and those who are convicted face up to life in jail.