Apple Daily newspaper founder Jimmy Lai was denied bail late Thursday, becoming the fourth high-profile Hong Kong dissident to be imprisoned this week.

Student leaders Joshua Wong, Agnes Chow and Ivan Lam were sentenced Wednesday to between seven months and 13.5 months in jail for their roles in the 2019 anti-extradition bill protest outside the Hong Kong police headquarters.

"China's brutal sentencing of these young champions of democracy in Hong Kong is appalling," said US official Nancy Pelosi, who met Wong and other activists in person earlier this year.

"This injustice is clear proof that Beijing will stop at nothing to stamp out dissent and to destroy the freedoms and real autonomy guaranteed to the people of Hong Kong," the Speaker of the House added.

A day later, Lai and two associates were arrested in the late hours of Wednesday evening by police on fraud charges related to the alleged misuse of the news group's office building.

While the pair of Next Media senior executives were released, the media mogul's application for bail was denied and he will be remanded to police custody until his next hearing in mid-April 2021.

The judge's decision to refuse bail over a lease infraction is highly unusual. Government prosecutors cited their fears that Lai might be a flight risk given his resources but while there is a trend of activists fleeing charges in Hong Kong, they are typically related to more serious offences.

It's an example of "classic Hong Kong 'lawfare'" according to Australian lawyer and writer Antony Dapiran. "How many pro-Beijing business people are happily engaged in similar fraud that will never be investigated?" he wrote in a Twitter post Thursday.

The prosecution has not ruled out additional charges under the National Security Law but needs more time to go through Lai's corporate finances.

Likewise, there is a possibility that Wong, Chow and Lam will see their sentences extended according to observers.

"To be honest, I have no idea when the trio could step out of the prison if Beijing pledges to impose the hardest charges on them arbitrarily," noted fellow Hong Kong activist Nathan Law who is now in exile overseas.