Hui Po-keung, a prominent cultural studies scholar, was on his way to take up an academic post in Europe before he was detained at the airport on Tuesday night, the sources said.

The other three arrested trustees of the fund include retired cleric Cardinal Joseph Zen Ze-kiun, veteran barrister Margaret Ng Ngoi-yee and gay rights activist and pop singer Denise Ho Wan-see. They were arrested on Wednesday evening, according to sources.

The sources said all four were arrested for "collusion with foreign forces", an offense under a new security law that Beijing imposed on Hong Kong in response to massive protests three years ago. The law has crushed dissent in the once outspoken business hub and can carry up to life in jail.

The fund has five trustees in total. The remaining one is the jailed democracy activist Cyd Ho Sau-lan. She is serving a 14-month jail sentence for involvement in different illegal assemblies in 2019.

"612 Humanitarian Relief Fund" helped arrested protesters pay for their legal and medical bills. The fund disbanded last year after the city's national security police demanded it hand over operational details, including information about its donors and beneficiaries.

Shortly before the fund closed, Hong Kong's Lingnan University confirmed that its contract with Hui had ended the month before but declined to state a reason on personal privacy grounds.

Academics who played prominent roles in Hong Kong's now largely decimated democracy movement have often found themselves dropped by universities and are struggling to find work.

An active social commentator and prolific author, Hui taught for more than two decades at Lingnan University and was credited by prominent former student leader Nathan Law for inspiring his political career.

Denise Ho was arrested one day after her 45th birthday. “I'm glad that I'm still intact and able to walk around freely.” she wrote in a Facebook post on Tuesday. She said she had no specific birthday wish, but hoped to share her birthday blessings and wishes to everybody.