Four students in Hong Kong have been arrested for "advocating terrorism," police said on Wednesday.

Senior Superintendent Steve Li said the arrests were made after several dozen Hong Kong University students passed a motion mourning the death of a 50-year-old who stabbed a police officer.

All four of those detained were men between the ages of 18 and 20, he added.

The arrests, made under the city's controversial national security law, could spark fresh concerns about freedom of expression in Hong Kong.

Authorities have described the attacker as a "lone wolf." He punctured the police's officer lung before stabbing himself in the chest.

The assailant died later in hospital, but the 28-year-old police officer survived.

He had been on duty on July 1 earlier this year for the anniversary of the former British colony's return to Chinese rule.

What did the students do?

The Hong Kong University student union passed a motion that expressed "deep sadness" for the attacker's "sacrifice."

The group's leaders have since withdrawn the motion, apologized and stood down from their posts.

But the resignations failed to satisfy police, which last month raided the union's offices, the campus TV station and the university's undergraduate office.

"[The motion] beautified, rationalized, glorified terrorism and an indiscriminate attack and encouraged suicidal acts," Li told reporters on Wednesday.

The 30 students who signed the text have also been banned from campus.

Advocating terrorism carries a sentence of up to 10 years in jail and is covered by Hong Kong's national security law.

The legislation also covers offenses deemed to be subversion, secession, terrorism and collusion with foreign forces.

What has been happening in Hong Kong?

Beijing imposed the national security law on the city last year after a series of huge pro-democracy protests.

The legislation has quickly criminalized much dissent, sparking a crackdown on freedom of speech and the free press.

In April, judges sentenced Hong Kong pro-democracy media tycoon Jimmy Lai, the founder of the Apple Daily tabloid, to 14 months in prison after being found guilty of unauthorized assembly.

It led to countries, such as the United States and the United Kingdom, to offer sanctuary to Hong Kong residents who wanted to flee.

Britain handed power over its former colony to China in 1997. The UK signed a treaty with China in 1984 in which Beijing pledged to respect Hong Kong's independence. Under the terms of the handover between Britain and China, both countries agreed Hong Kong should have its own legal system.

They also agreed that fundamental rights such as freedom of assembly and free speech would be guaranteed for 50 years.

The "one country, two systems" principle — named so because such rights do not exist elsewhere in China — is enshrined in Hong Kong's Basic Law. Many activists in Hong Kong as well as in exile have said the principle is no longer recognized by Beijing.

jf/sms (AFP, Reuters)