Hong Kong police have arrested an organiser of annual vigils for the victims of China's1989 Tiananmen crackdown on pro-democracy protesters, in what activists see as a suppression of one of the city's most powerful symbols of democratic hope.

Hours after the arrest of Chow Hang Tung, vice-chairwoman of the Hong Kong Alliance in Support of Patriotic Democratic Movements of China, police cordoned off large swathes of Victoria Park, the main venue for the annual vigils.

Thousands of officers are expected to patrol the city's streets to prevent gatherings.

The heightened vigilance from authorities was a marked departure from Hong Kong's cherished freedoms of speech and assembly, bringing the global financial hub closer in line with mainland China's strict controls on society, activists say.

Hong Kong's annual 4 June vigil, the world's largest, is widely seen as a symbol of the former British colony's democratic aspirations and desire to preserve its different way of life from mainland China.

"She only wanted to go to Victoria Park, light a candle and commemorate," Chiu Yan Loy, Executive Member of the Alliance, told Reuters, adding he believed Chow's arrest was meant to strike fear into those planning to attend the vigil.

Police, which banned the vigil for a second year in a row, citing the coronavirus, confirmed they arrested a 36-year-old Alliance member and a 20-year-old food delivery man for promoting an unauthorised assembly.

Police later said the decision to close most of Victoria Park was motivated by social media calls on people to rally despite the ban.

"From the bottom of my heart, I must say I believe Hong Kong is still a very safe and free city," senior superintendent Liauw Ka-kei told reporters, adding that police had no option but to enforce the law.

Ms Chow told Reuters this week before her arrest that 4 June was a test for Hong Kong "of whether we can defend our bottom line of morality."

"As long as they haven't said candles are illegal, we will light a candle," she said.

Her Facebook page said Ms Chow will mark the anniversary by fasting if she is unable to light a candle due to her arrest.

The Alliance's chairman Lee Cheuk-yan is in jail over an illegal assembly.

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Police did not say whether commemorating Tiananmen would breach a sweeping national security law China imposed in 2020 to bring its most restive city onto an authoritarian path.

City leader Carrie Lam has only said that citizens must respect the law, as well as the Communist Party, which this year celebrates its 100th anniversary. June 4 commemorations are banned in mainland China.

Last year, thousands in Hong Kong defied the ban, gathering in the park and lining up on sidewalks with lit candles across the city, in what was largely a solemn event, bar a brief scuffle with police in one district.

Prominent activist Joshua Wong received a 10-month prison sentence last month for participating in last year's vigil, while three others got four-to-six-month sentences. Twenty more are due in court on June 11 on similar charges.

"It is a battle against oblivion," exiled activist Sunny Cheung told Reuters by text.

Many plan to light candles again in their neighbourhood, if safe to do so. Activists online called for people to turn on the lights in their homes at 8.00 pm. Some churches will be open for prayers.

Jailed activist Jimmy Sham said via his Facebook page he planned to "light a cigarette at 8pm."

"We do not see the hope of democracy and freedom in a leader, a group, or a ceremony. Every one of us is the hope of democracy and freedom."

In democratically-ruled Taiwan, a memorial pavilion will beset up in Taipei's Liberty Square, where people can lay down flowers while following social distancing.

Taiwan President Tsai Ying-wen said the island's people will never forget what happened 32 years ago.

China has never provided a full account of the 1989 violence. The death toll given by officials days later was about 300, most of them soldiers, but rights groups and witnesses say thousands of people may have perished.