Hong Kong police detained one of the organisers of the annual vigil commemorating Beijing's deadly Tiananmen Square crackdown as authorities sought to prevent any show of pro-democracy people power on Friday's sensitive anniversary. About 7,000 officers have been placed on standby to stamp out any attempt to hold a mass candlelight vigil that Hong Kongers have attended in their thousands each anniversary for the past three decades. Chow Hang-tung, one of the few remaining prominent democracy activists not already in jail or in exile, was detained by four police officers outside her workplace on Friday morning. Ms Chow, 37, is one of the vice chairs of the Hong Kong Alliance which organises the annual vigil. A police source said she had been detained on suspicion of publicising an unlawful assembly. Huge crowds have traditionally gathered in Hong Kong to mark the anniversary of Chinese troops crushing peaceful student-led democracy protests in Beijing's Tiananmen Square on June 4, 1989. Hundreds were killed in the crackdown, by some estimates more than 1,000. Public commemorations of the event are forbidden on the mainland. Under a "One country, two systems" policy that was meant to give Hong Kong more freedoms, the city was the only place on Chinese soil where large-scale commemorations were tolerated. Large crowds massed each year in Victoria Park. But, following months of huge and often violent democracy protests in 2019, also led largely by students, China has carried out a sweeping campaign to silence dissent and enforce "patriotism". Authorities banned this year's gathering citing the coronavirus pandemic – although Hong Kong has not recorded an untraceable local transmission in more than a month. While last year's vigil was also denied permission because of the pandemic, thousands defied the ban. Authorities have also warned in recent days that the subversion clause of a powerful new national law imposed on Hong Kong could be used against those marking the Tiananmen anniversary. Beijing imposed the law just a few weeks after last year's Tiananmen rally and it has transformed the city's once freewheeling political landscape. More than 100 pro-democracy figures have been arrested under the security law, mostly for political views and speech. Most are denied bail and face up to life in prison if convicted. However China has been unable to quash all dissent, and Hong Kongers were planning creative ways to mark the anniversary this year. "A regime can ban an assembly but it can never ban the indelible grievances in people's hearts," Lee Cheuk-yan, a jailed democracy activist and the current head of the Hong Kong Alliance, wrote on his Facebook page on Thursday. Activists have called on residents to light candles in their homes or neighbourhoods on Friday evening, or post commemoration messages on social media. "Turn on the lights wherever you are – be it torchlight on your phone, real candles or electronic candles," Ms Chow wrote on Facebook page just hours before her arrest. Clara Cheung was among a small group of artists who gathered near Victoria Park on Thursday evening. She brought 64 white flowers, representing June 4, and laid them on the street. "We need to find a new way to express ourselves," she said. Pro-Beijing politicians have suggested that calls to "End one-party rule" and "Bring democracy to China" – both common chants at Tiananmen vigils – could now be deemed subversion, one of the crimes in the broadly worded national security law. The security legislation has also been combined with a new campaign dubbed "Patriots rule Hong Kong", aimed at purging anyone perceived to be disloyal from public office. In mainland China, the Tiananmen anniversary is usually marked with an increase in online censorship and the square in Beijing being cordoned off. There was tight security at the square on Friday, with police checking the IDs of people at each point of access. China often faces international criticism for its campaign to stifle remembrance of the crackdown. On Friday, US Secretary of State Antony Blinken called "a full accounting of all those killed, detained, or missing". Taiwan's foreign ministry said "the students of Tiananmen and Hong Kong must not be erased nor forgotten".