Hong Kong has fallen into an era of "white terror", democracy campaigners said Thursday after four students –- one aged just 16 –- were arrested for social media posts deemed to be a threat to China's national security.
Wednesday's arrests were the first made by Hong Kong police's new national security unit, which was set up after Beijing imposed a controversial law last month aimed at quashing the city's democracy movement.
The arrests added to a deepening sense of dread in Hong Kong that its cherished freedoms on speech -- once thought to be guaranteed under a "One China, Two Systems" model -- are on the way to being eviscerated.
The four arrested were former members of Student Localism, a pro-independence group that announced it was disbanding the day before the new national security law was enacted.
Police said the three males and one female -- aged between 16 and 21 -- were arrested on suspicion of organising and inciting secession through comments made on social media posts after the law came in.
"They wanted to unite all the independent groups in Hong Kong for the view to promote the independence of Hong Kong," Li Kwai-wah, from the police's new national security unit, told reporters.
Footage posted online showed plainclothes police leading Tony Chung, the 19-year-old former leader of Student Localism, being detained with his hands tied behind his back.
Student and rights groups condemned the arrests, saying they heralded the kind of political suppression ubiquitous on the authoritarian Chinese mainland.
"Hong Kong has fallen into the era of white terror," the Student Unions of Higher Institutions, which represents 13 student unions, said in a statement overnight.
"It is crystal clear that more and more Hongkongers (will) have to endure… Communist terror," it added.
Nathan Law, a democracy campaigner who went into exile after the law was imposed, expressed similar sentiments on Twitter.
"White terror, politics of fear dispersed in Hong Kong," referencing a Chinese idiom to describe political persecution.
In an overnight statement, Hong Kong police warned people could commit crimes by what they write online.
"Police remind the public that the cyber world of the Internet is not a virtual space beyond the law," the force said.
"Anyone who commits an unlawful act, whether in the real or in the cyber world, is liable to criminal prosecution."
Sophie Richardson, a China expert with Human Rights Watch, said Beijing's new legislation was being wielded against peaceful political speech.
"The gross misuse of this draconian law makes clear that the aim is to silence dissent, not protect national security," she said.
The security law gave China's Community Party rulers far more direct control over Hong Kong, which was supposedly guaranteed 50 years of freedoms as part of 1997 handover from Britain.
But last year the city was rocked by seven straight months of huge and often violent pro-democracy protests.
Beijing said the national security law was needed to end that unrest and restore stability, describing it as a "sword" hanging over the heads of lawbreakers.
It targets four types of crime: subversion, secession, terrorism and colluding with foreign forces -- with up to life in prison.
Critics, including many western nations, say it has demolished the "One Country, Two Systems" model.
The law bypassed Hong Kong's legislature and its details were kept secret until the moment it was enacted.
It empowers China's security agents to operate openly in the city for the first time.
Beijing has also said it will have jurisdiction for especially serious cases, toppling the legal firewall that has existed since the handover between Hong Kong's independent judiciary and the Chinese mainland's party-controlled courts.
China has also claimed it can prosecute anyone anywhere in the world for national security crimes.
On the mainland Beijing routinely uses similar national security laws to crush dissent.
The first arrests in Hong Kong came a day after the law was enacted against people who possessed pro-independence flags and slogans critical of Beijing -- including a 15-year-old girl.
At least 15 people have now been arrested under the new law since it was enacted on June 30.