HONG KONG — "There is no 'red line'," in Hong Kong any more, says a 30-something humanities professor in the city.

"If they want to come after you, everything can be used as an excuse." He did not wish to reveal his name because of the repercussions that could follow.

He says his nightmare is being named and attacked by Beijing-backed media, which could cost him his job, or worse, his freedom. That fear has swept through Hong Kong's universities and academic circles, which once attracted top talent. The city was close to the mainland, yet far enough to host progressive classrooms, world-class libraries and archives that allowed academic freedom, even in Chinese studies.

But that is no longer the case, academics and students tell the BBC, many choosing to stay anonymous out of fear. In the academic year 2021/22, more than 360 scholars left Hong Kong's eight public universities. The turnover rate — 7.4% — is the highest since 1997, when Hong Kong returned to Chinese rule, according to official data. Foreign student enrolments have dropped by 13% since 2019.

"The free atmosphere that existed is gone and people are worried," says Stephan Ortmann, a political scientist at the Hong Kong Metropolitan University. He says many of his colleagues have left and those that remain are wary — he has heard of teachers who have removed all Hong Kong and China-related material from their courses.

The self-censorship, academics say, began after the National Security Law (NSL) took effect in 2020. The sweeping legislation targets any behaviour deemed secessionist or subversive, allowing authorities to target activists and ordinary citizens alike.

Beijing says the law, which it pushed through following massive anti-government protests in 2019, has ushered Hong Kong "from chaos to governance". But it has also transformed this once-vibrant city. There used to be protests, big or small, nearly every weekend — but now open displays of dissent are unimaginable. Public libraries have been emptied of books promoting what officials call "bad ideologies" and films are censored on national security grounds. Pro-democracy activists were barred from "patriot-only" local elections held over the weekend — but the most prominent of them, some of whom are also academics, are either in jail or exile.

At the entrance to the sprawling, hilly campus of the Chinese University of Hong Kong (CUHK), seven security guards are manning a booth where teachers, students and visitors must identify themselves.

The security checks were introduced in 2021 — the same year that two influential pro-democracy media outlets, Apple Daily and Stand News, and dozens of rights groups and trade unions were shuttered.

"It feels very different," says an alumnus who was visiting campus that day.

CUHK was a battleground in 2019, with black-clad protesters and riot police trading petrol bombs, bricks, tear gas and rubber bullets. It bears no signs of dissent now. The democracy wall, which was once filled with pro-democracy posters and post-it notes, has been stripped bare and barricaded. The statue of the Goddess of Democracy, which was erected in memory of the thousands who died in Beijing's Tiananmen Square crackdown in 1989, is gone. It was removed in the early hours of Christmas Eve in 2021.

"My friends and I definitely feel a sense of helplessness," says a CUHK student who didn't want to be identified. "I chose to study social sciences partly because of the social movement... I want to learn more and contribute more. But now fewer things can be done."

He has begun avoiding potentially sensitive courses, such as those on Chinese politics and history. He is also worried that the research papers he writes can be leaked, despite the university's guarantee to protect students' privacy. His anxiety is not unfounded because Hong Kong now has a hotline where people can report others for breaching the NSL.

He is uncertain what future the city he calls home holds for him. He knows friends who quit university a year after enrolling and joined 140,000 other HongKongers to move to the UK on special visas that allow them to live and work there. Others are planning to leave soon, he says.

"Hong Kong was on an upward trajectory to become a centre of academic excellence outside the West," says a China expert who left Hong Kong after 2020. "It is heart-breaking that 20 years of progress was broken down singlehandedly [by the NSL]."

Dr Ortmann, who arrived in 2011, is just as disappointed about Hong Kong losing its status as a centre for scholarship on China: "It provided unparalleled access to many of the sources. It has definitely become less important as access to many sources has disappeared."

Even being an expert on China makes you a target, he says. "A colleague of mine, who is a China scholar, was kept at the border for about four hours, when he entered Hong Kong from mainland China."

"They didn't like us for a long time, but they made a move against us after 2019," says a humanities scholar who left the city two years ago, after he was denied tenure — a permanent job in academia — despite recommendations from various peers.

Job applications from professors and scholars overseas have dried up, says one social sciences teacher, adding that even hiring research assistants has become difficult.

Few students of humanities or social sciences in Hong Kong are now enrolling in PhDs, and the chances that those who do will finish the programme are diminishing.

Historian Rowena He first arrived in Hong Kong in July 2019 in the early months of the protests. She knew it could be risky but she took the chance.

Hong Kong was a beacon for the Guangzhou-born scholar of the 1989 Tiananmen Square protests. She grew up in Canada on Hong Kong dramas, and watched the city's journalists report on what happened in and around Tiananmen Square in 1989.

This October, she found out her application to renew her Hong Kong visa had been rejected, after a year-long wait. She was fired by her employer, CUHK, days later. Hong Kong's leader John Lee said Prof He's visa was denied as part of a regular process that screened for those who could "cause security and criminal risks".

"I just feel sad for the city and the people," she says. "This would have serious, broader implications... people would be asking, 'Can I still continue my academic work in Hong Kong? What can I teach?'"

She says hers has been a rather "lonely journey", given public discussion about the student-led protests of 1989, and the bloody crackdown that followed, was forbidden in China. Even the internet carries no trace of that seminal moment.

But this only reinforced her bond with Hong Kong: "I know, no matter what, every year on 4 June, tens of thousands of people would go to Victoria Park. Together, they hold these candles and tell the world that we are still here, we still care, we still remember."

Hong Kong, which had always staged the largest candlelight vigil in honour of the 4 June massacre, held the last one in 2019, after which it was banned.

Prof He says she kept "as low a profile as possible" at CUHK, but she didn't censor her classes. "I refused to use 'controversial' or 'sensitive' to describe what I am working on. My basic responsibility is to teach historical truth and universal values." She dined with her students every week so they could talk about what was happening in the city.

"We were able to support each other even we were living in fear."

Then in February, the Hong Kong state-owned and Beijing-backed newspaper Wen Wei Po called her an agent of the West.

As an expert on political exile, she thought she knew enough about the experience. "I thought I understood them very well. When it came to the time that I had to carry just one suitcase with all my clothing and move from one Airbnb to another, I started to feel that I actually did not."

The warmth and support of HongKongers, at home and abroad, consoled her.

"There will be a time of darkness. But civil society would die only when its own people give up. We should not give up on Hong Kong." — BBC