Hong Kong authorities have sealed off a park where many thousands gather annually to commemorate China's 1989 Tiananmen crackdown, and police arrested the vigil's organiser.
Activists see the move as a suppression of one of the city's main symbols of democratic hope.
With a security blanket deployed on Friday to prevent people gathering to light candles for the pro-democracy demonstrators killed by Chinese troops in Beijing 32 years ago, some marked the anniversary in churches or at home amid fears of being arrested.
In the working class district of Mong Kok, minor scuffles broke out as tensions flared, while police arrested one person for misconduct in a public place.
Police conducted stop-and-search checks across the city, including at three cross-harbour tunnels leading to Hong Kong island, causing long delays during the evening rush hour.
Water cannons 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.
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.
Authorities warned of more arrests and said that anyone who took part in an unauthorised assembly could face up to five years in jail.
"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.
Hong Kong chief executive Carrie Lam has said 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.
China has never provided a full account of the 1989 violence in Beijing's Tiananmen Square. 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.