Lawyer-turned-activist Chow Hang-tung, who helped organise an annual vigil to mark Beijing's 1989 Tiananmen Square crackdown, has been behind bars since 2021 and could face life in prison over a separate subversion trial later this year.

Defending herself in court on Wednesday, her sneakers and beige coat contrasting sharply with the other lawyers in wigs and gowns, she gave Hong Kong's chief justice and others a rare scolding.

"A police state is created with the complicity of the court in endorsing (the government's) abuses. Such complicity must stop now," she said.

Hong Kong's Court of Final Appeal has seen an exodus of foreign judges since Beijing passed a sweeping national security law on the former British colony in 2020, following huge and often violent pro-democracy protests the year before.

British judge Jonathan Sumption resigned last year after saying Hong Kong's "rule of law is profoundly compromised", leaving six overseas jurists and eight local ones at the top court.

Chow on Wednesday said city authorities were "treading on law's dignity to bolster the police's authority".

Chow was a leader of the now-disbanded Hong Kong Alliance, which organised the city's annual candlelight Tiananmen vigil.

Hong Kong used to be the only place on Chinese soil where people could publicly mourn the deadly clampdown on pro-democracy demonstrators in Tiananmen Square on June 4, 1989, but commemorations have gone underground in recent years.

The Wednesday hearing was part of a three-year legal battle after Chow and two colleagues refused to turn over information on the Alliance's members and finances when police accused it of being a "foreign agent".

Prosecutors said it was enough for the police chief to have a "reasonable belief" that the group counted as a foreign agent, without needing to prove it in court.

"A police state is where the police are free to accuse anyone of being a foreign agent and the court is obliged to defer to that judgement even if that is clearly wrong," Chow said Wednesday.

"We are not concerned with what is a police state," the chief justice replied.

He and other judges tried to steer Chow towards more technical arguments, while also expressing scepticism towards the government's position later in the hearing.

A smiling Chow was greeted with applause from the public gallery as she was led away by guards.

The court will rule at a later date.