Another 19 people were slapped with fixed penalty tickets for allegedly flouting social distancing rules.
In a series of Facebook posts, the force said there were people gathering in Causeway Bay in the afternoon, chanting, displaying flags and protest placards, and attracting the attention of onlookers.
Three arrests were made after warnings issued by officers were ignored, the force said.
“One man and two women were arrested on suspicion of disorderly conduct in a public place. The male suspect was also arrested after he allegedly failed to produce proof of identification,” police said.
Prominent democracy activist Alexandra Wong, who is known as Grandma Wong, was seen waving a British flag in Causeway Bay before she was taken away by officers. It was not clear whether she was one of those arrested.
The force warned that officers were stepping up patrols and intercepting “suspicious” people, to prevent and combat crime.
Another man had been arrested on suspicion of possessing an imitation firearm nearby, with officers condemning the alleged incident.
Meanwhile, the pro-democracy League of Social Democrats set up a street booth near the Times Square shopping mall, under the watchful eye of the police.
The group had wanted to set the booth up in Great George Street, but police had cordoned off the road.
In Mong Kok, police said 11 people were arrested on suspicion of distributing seditious publications, including members of the group Student Politicism.
The group said on social media that its convenor Wong Yat-chin, as well as two members and a volunteer had been arrested as they were trying to man a street booth.
Wong had also been arrested over a street booth in the same area on June 4.
The Confederation of Trade Unions (CTU) managed to set up two booths in Mong Kok to distribute leaflets and let people write down messages. The booths were soon surrounded by officers, who recorded the personal details of those present.
The CTU said with the July 1 march banned this year, it believes the rights of Hongkongers are being eroded and it wanted people to be able to write down their thoughts, despite the “white terror” in society.
Last updated: 2021-07-01 HKT 21:26