Hong Kong authorities on Saturday detained multiple people as they pounced on any attempt at public commemoration of the 33rd anniversary of the Tiananmen crackdown, as China vies to remove all reminders of the deadly event.

Discussion of June 4, 1989, when China set troops and tanks on peaceful protestors, is all but forbidden on the mainland.

Semi-autonomous Hong Kong had been the one place in China where large-scale remembrance was still tolerated -- until two years ago when Beijing imposed a national security law to snuff out dissent after huge pro-democracy protests in 2019.

AFP reporters saw at least half a dozen people being taken away by police on Saturday, the majority in the evening, including activist Yu Wai-pan from the League of Social Democrats (LSD) party.

LSD said Yu was later released without charge, while fellow member Lau Shan-ching was arrested for wearing a shirt with a portrait of late Chinese democracy activist Li Wangyang with a mask that read "mourn June 4".

Police confirmed that an 80-year-old man was arrested for obstructing officers earlier in the day, but have yet to confirm the number of arrests made after nightfall.

Authorities had warned that "participating in an unauthorised assembly" on Saturday risked the maximum penalty of five years' imprisonment.

They also closed large parts of Victoria Park, once the site of packed annual candlelight vigils.

The park and adjacent Causeway Bay shopping district were heavily policed Saturday, with multiple people targeted for searches.

Yu and two other LSD members, all wearing white masks with a black cross across the mouth, came to Causeway Bay in the evening and stood silently on the street.

Within 30 seconds, police had taken them away for a search.

They were released but as they approached Victoria Park they were stopped and Yu was taken away.

Causeway Bay shopping district -- one of Hong Kong's busiest shopping hubs -- was heavily policed all day
Causeway Bay shopping district -- one of Hong Kong's busiest shopping hubs -- was heavily policed all day AFP / Peter PARKS

"For 33 years it has always been peaceful, but today it's like (police) are facing a big enemy," Chan Po-ying, head of the LSD, said.

"The candlelight will not go out; the hearts of people will live on."

Near the park in the evening, dozens of scattered people turned on their phone lights.

Over a megaphone, police said to turn them off, warning the people they risked breaching the law on unauthorised assembly.

When asked why that would constitute a crime, an officer told AFP he would "leave it to my colleagues to explain in a press conference".

Earlier, police had also told people turning on LED candles to desist.

Police searched one man for over 20 minutes and then told him to leave.

"They're even afraid of an old person like me, I'm over 60," the man, surnamed Chan, said. "Hong Kong is already dead."

Others were stopped and searched for carrying flowers, wearing black and in one case, carrying a toy tank box.

Some people left candles in phone booths or on street corners, or distributed small stickers with candles drawn on them.

"We can't make a big fuss, but there are still small ways... to tell everyone they are not alone," one young woman told AFP.