Hong Kong national security police arrested six people across the city on Tuesday under a new security law for alleged seditious intent, including a pro-democracy barrister already behind bars.

Security Secretary Chris Tang told reporters the six were suspected of using a Facebook page to "advocate hatred" against the governments in Hong Kong and China, which took back control of the former British colony in 1997, and against the judiciary.

He said one of those arrested was Chow Hang-tung, a prominent barrister and pro-democracy activist who has been detained since September 2021 at a maximum security women's prison.

These were the first arrests under a new batch of national security laws that were passed by the city's pro-Beijing legislature in March despite international criticism by countries including the U.S. that they could damage Hong Kong's international financial hub credentials.

Britain handed Hong Kong back to China under a "one country, two systems" formula which guaranteed its freedoms, including freedom of speech, for 50 years.

The arrests come despite criticism that an ongoing crackdown since 2020 that has silenced dissent and shut down liberal media outlets and NGOs has sapped Hong Kong's dynamism and hurt the city's economic prospects.

The Hong Kong and Chinese governments say the national security legislation has restored stability to the city after months of sometimes violent pro-democracy street protests in 2019.

Later this week, the city's high court is expected to deliver a landmark verdict that will decide the fate of 16 leading Hong Kong democrats that could see them jailed for an alleged plot against the government.

Chow is alleged to have used the five others to "publish posts with seditious intent" focused on an "upcoming sensitive date", the statement added. It also alleges they had the "intention of inciting netizens to organise or participate in illegal activities in a later period".

The offence carries a maximum jail term of seven years.

Local media cited sources as saying the "sensitive date" was June 4, the anniversary of a bloody crackdown on Chinese pro-democracy protesters in and around Beijing's Tiananmen Square in 1989.

Those arrested were between 37 and 65, but Tang did not name the other five.

Chow was formerly one of the leaders of the Alliance - a pro-democracy group that used to organise an annual candlelight vigil to commemorate June 4 in a downtown park.

Chow is currently serving several jail terms, including one linked to unauthorised assembly convictions for June 4 commemorations in recent years.

The Alliance was forced to disband under a security crackdown, and the vigils were banned.

"Those intending to endanger national security should not delude themselves into thinking they can avoid police pursuit by remaining anonymous online," the statement added.

Chow also faces legal proceedings for another national security offence, "incitement to subversion", which carries a sentence of up to 10 years imprisonment, alongside two former Alliance leaders Albert Ho and Lee Cheuk-yan.

(Reporting by James Pomfret and Jessie Pang; Editing by Andrew Heavens, Kevin Liffey and Nick Macfie)