Lawyer Chow Hang-tung - who was jailed in January - will remain in custody as she faces two other charges under the city's national security law.

But she won her appeal against her "unauthorized assembly" conviction on Wednesday,

A judge ruled the police hadn't properly justified the vigil's ban.

Hong Kong authorities have cited Covid restrictions as the reason for its ban on Tiananmen vigils since 2020.

The city used to be one of the only sites on Chinese territory where authorities allowed tributes.

Ms Chow was arrested in June 2021 for "inciting" the public to take part in that year's vigil. She had led the Hong Kong Alliance, a group which had organized the annual demonstrations and that year she posted articles on social media and on news sites urging Hong Kongers to turn out or light a candle in tribute.

At her trial in January this year, she was jailed for encouraging the assembly in breach of Covid laws.

Magistrate Amy Chan said she had been "self-righteous" in "completely disregarding the law to think that the freedom of assembly was more important than public health".

However a High Court judge on Wednesday found police had not properly explored the options for how the demonstration could have gone ahead in a Covid-safe manner.

Judge Judianna Barnes said police "did not seriously consider" other health measures, thus ignoring a requirement in the law that public meetings should not be banned if they can be safely facilitated.

The ruling could have legal implications for other Hong Kongers jailed for taking part in Tiananmen vigils. They include the pro-democracy tycoon Jimmy Lai, who was sentenced to 13 months for the alleged offense a year ago.

Lai was jailed for nearly six years on fraud charges earlier this month and faces the prospect of life behind bars due to a separate trial on national security charges, which is due to begin next September.

Ms Chow and other human rights advocates have long argued that Covid restrictions were just an excuse for Hong Kong authorities to ban commemorations of the Tiananmen protests - a heavily censored and highly sensitive topic in mainland China.

In previous years thousands of people gathered to remember the victims of the crackdown on 4 June 1989, when the Chinese military attacked pro-democracy protesters camped in Beijing's Tiananmen Square, killing an unknown number of civilians.

However China has asserted its rule over Hong Kong since major protests in 2019 protesting against Beijing's influence and the rolling back of civil rights in the city.

Since then public commemoration of the Tiananmen protests has been targeted by the authorities. Last year monuments marking the event were removed from university campuses and a museum was also shut down.