Hong Kong prosecutors on Wednesday gained new powers to challenge High Court judges' decisions to acquit people in national security trials, which allows for defendants to be re-arrested and detained even if they are found not guilty.

Dozens of activists and opposition figures are awaiting trial under Hong Kong's national security law, which was imposed by Beijing in 2020 after the finance hub saw huge and sometimes violent pro-democracy protests.

Prosecutors have so far secured a 100 percent conviction rate under the security law, but officials argued that the rule change was needed to plug legal loopholes.

The new rules "prevent possible cases of miscarriage of justice", secretary for justice Paul Lam said Wednesday after the bill passed without opposition in the city's legislature -- which is stacked with Beijing loyalists.

A former British colony, Hong Kong follows the common law tradition that features jury trials and rules that forbid a court from trying a person twice for the same crime.

Under the new rules, if a security trial defendant is acquitted by the High Court, prosecutors can challenge the ruling and ask for a retrial.

If the appeal is granted, the case will be reheard at the High Court level, with the new verdict overriding the previous one.

Previously, prosecutors were barred from appealing certain types of acquittals. Defendants have the right to appeal their conviction and sentence at appellate courts.

Beyond national security cases, the amendment also affects ordinary criminal cases.

Under the new rules, if a High Court judge rules that a defendant can walk free because the case against them is too weak, prosecutors will now be permitted to contest those acquittals -- which are known as "no case to answer".

This will trigger a retrial if their challenge succeeds.

The new amendments also impose restrictions on media reporting on the appeal "to ensure a fair trial", though the limits could be "relaxed".

In an interview with Hong Kong's official broadcaster in May, Lam repeatedly denied that the government was stacking the deck against defendants in national security cases.

"We are simply trying to align the mechanism for High Court cases with those handled in lower courts," he said.

Government officials say defendants' rights to appeal remain intact, even with the amendments.

Under the national security law's stringent rules for bail, most defendants have been kept in custody even if their trial was not slated to begin for months.

Simon Young, a legal scholar at the University of Hong Kong, told AFP that the amendments meant "there would be a concern about the extended detention of the defendant" if their acquittal is appealed by prosecutors and they must wait for a retrial.

Hong Kong has so far convicted 79 people under the national security law.

The High Court is currently trying 47 pro-democracy figures for "conspiracy to commit subversion" -- the city's largest security trial yet.

The security trial of jailed media tycoon Jimmy Lai and executives at his now-shuttered newspaper Apple Daily is expected to start later this year.

Both cases are being tried without a jury, upon request by the prosecution.