HONG KONG (AFP) - Hong Kong democracy activist Agnes Chow on Tuesday (Sept 1) warned that dissent was being silenced in the financial hub as she reported to the police after her recent arrest under China's new security law for the city.

The 23-year-old was one of the first opposition politicians to be detained under the law for alleged "collusion with foreign forces" - and could face up to life in jail if convicted.

"I hope the world, not only the whole of Hong Kong... could know that the national security law is actually not a legal thing, but a political tool for the regime, for the government to suppress political dissidents," Ms Chow told media outside a police station.

Ms Chow, who has a large following in Japan because she speaks Japanese, was arrested on Aug 10 in the largest police operation since the sweeping law was imposed on the restless city in late June.

During that operation, more than 200 officers raided the offices of local newspaper Apple Daily and arrested its owner Jimmy Lai - a prominent critic of Beijing.

Six of the 10 people arrested that day, including Mr Lai and Ms Chow, were detained under the security law.

Police alleged that Apple Daily executives used an overseas account to provide financial support for a three-member group that lobbied for foreign sanctions against Hong Kong - an act now illegal under the new security law.

That day, police also paid a discreet visit to the local newsroom of Nikkei, a Japanese financial newspaper, armed with a court order.

The order was in relation to an ad placed in Nikkei a year ago calling for international support for pro-democracy protests then convulsing Hong Kong.

On Tuesday, Ms Chow confirmed the ad came up during her questioning by police.

"If this advertisement published in 2019 is really the evidence that I was against the national security law in 2020, it will be really ridiculous," she said.

The security law, which came into effect on June 30, has no retrospective effect, according to a Hong Kong police spokesman, and police say the group continued to operate after the law came into effect.

Those arrested have denied those allegations.

Ms Chow has not been officially charged by the police and will have to report to the police station on Dec 2, one day before her 24th birthday.

The new law removes the presumption of bail unless a judge has sufficient reason to believe a defendant will not continue to endanger national security.

So far, the only person charged under the law has been rejected bail.

"My only wish is that I hope... I could go back to my home after the report and I could have my 24th birthday at home," Ms Chow said.