Hong Kong sealed off a park where tens of thousands gather annually to commemorate China's 1989 Tiananmen crackdown and arrested the vigil's organiser on Friday, in what activists see as suppression of one of the city's main symbols of democratic hope.
A blanket of security was thrown over the former British colony to prevent people gathering to light candles for the pro-democracy protesters killed by Chinese troops in Beijing 32 years ago.
Media reported police were conducting stop-and-search checks at three cross-harbour tunnels leading to Hong Kong island, causing long tailbacks during the evening rush hour. Water cannon and armoured vehicles were spotted.
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 June 4 vigil, the world's largest, is usually held in Victoria Park, to the east of Hong Kong's central business district, and is widely seen as a symbol of the semi-autonomous city's democratic aspirations and desire to preserve its distinct way of life.
This year's anniversary is the first under a sweeping and contentious national security law Beijing imposed on its freest city last year.
Early on Friday, police arrested Chow Hang Tung, vice-chairwoman of the Hong Kong Alliance in Support of Patriotic Democratic Movements of China, for promoting an unauthorised assembly. Hours later, officers cordoned off most of the downtown park, including football pitches and basketball courts.
"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 her 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, said there were still social media calls for people to rally despite the ban and warned of more arrests. Anyone who took part in an unauthorised assembly could face up to five years in jail, the Security Bureau said.
"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.
Some activists have questioned why the vigil was banned when concerts, art fairs and other events have been allowed to go ahead and life in the city - which recorded just one new coronavirus case on Thursday - has largely returned to normal.
Chow told Reuters this week before her arrest that June 4 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 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.
Police did not say whether commemorating Tiananmen would breach the new national security law.
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 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. Activists online called for people to turn on the lights in their homes at 8.00 p.m. Some churches will be open for prayers.
Students at Hong Kong University performed the annual ritual of cleaning the Pillar of Shame statue in the campus, an eight-metre-tall monument commemorating Tiananmen.
"We want to defend the historical truth," student union president Charles Kwok said.
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."
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.
