At 7pm, police cordoned off Exit E of the Causeway Bay MTR station, and reopened it minutes later.

Police also hoisted the purple flag an hour later, warning citizens that the slogan they are yelling might breach the national security law.

Members of the public were heard chanting slogans, including “No Rioters, Only Tyranny”, and “Vindicate June 4”, a slogan used for years demanding the vindication of those who perished during the 1989 Tiananmen Square crackdown.

Meanwhile, the purple flag was also hoisted on Sai Yeung Choi Street South in Mong Kok after people gathered in the vicinity, with some heard chanting the slogan "Liberate Hong Kong, revolution of our times", a slogan now deemed illegal by the government that connotes separatism or subversion.

This came as an appeal board upheld police’s ban on the annual June 4 candlelight vigil to commemorate the 1989 incident citing public health concerns.

Police had cordoned off the football pitches, basketball courts and central lawn in Victoria Park citing the Public Orders Ordinance since 2pm today.

Senior superintendent Liauw Ka-kei said the police are aware that online calls were still being made to defy police’s ban and participate in the unlawful activity.

“We believe that the banned rally will still take place, therefore decided to cordon off those areas due to public security concerns,” Liauw said.

“The closure will be lifted as soon as circumstances allow, but if the situation worsened, we might have to close down more areas in the park,” he added.

Liauw also said the operation is to prevent illegal behaviors from happening, and therefore there will not be an issue if citizens act in accordance with the law.

Heavy police presence can already be seen in the afternoon, as hundreds of police officers stood alert near Victoria Park, and around 20 police vehicles can also be sighted in the vicinity.

Later in the afternoon, the Security Bureau said in a statement that no one should take part in or advertise or publicise an unauthorised assembly.

"The public should not defy the law. If anyone attempts to challenge the law, the Police will take resolute enforcement action according to the law in a serious manner," the statement wrote.