Police deployed across Hong Kong on Thursday to stamp out any sign of dissent as China's Communist Party celebrated its centenary in a vivid illustration of how the once-outspoken finance hub has been successfully muzzled.

The grand celebrations in Beijing coincided with the 24th anniversary of Hong Kong's 1997 handover to China by colonial Britain.

Since 2003, Hong Kongers have regularly filled the streets each handover anniversary with marches and rallies.

John Lee praised China's imposition of a sweeping national security law in Hong Kong
John Lee praised China's imposition of a sweeping national security law in Hong Kong AFP / Peter PARKS

But protests have been all but banned in Hong Kong as China carries out a sweeping crackdown against its opponents after huge and often violent pro-democracy rallies convulsed the city two years ago.

More than 10,000 officers -- one-third of the city's force -- were deployed to prevent any public gatherings as officials warned that any attempt to assemble would lead to prosecution.

The city's main park was cordoned off and police said 19 arrests were made throughout the day.

Hong Kong Police officers goose-step carrying the Chinese and Hong Kong flags during a flag-raising ceremony
Hong Kong Police officers goose-step carrying the Chinese and Hong Kong flags during a flag-raising ceremony AFP / Peter PARKS

Local media said a police officer was stabbed on Thursday evening near Victoria Park, although it was not immediately clear who carried out the assault or why.

The assailant was arrested at the scene. He had also stabbed himself after the attack, according to local media reports citing police sources.

Hospital Authority confirmed that the suspect was certified dead at 23:20 pm after being taken to hospital.

With Hong Kong's Chief Executive Carrie Lam attending the festivities in Beijing, her deputy John Lee oversaw a flag-raising ceremony as police and water cannon trucks patrolled nearby.

Raphael Wong holds a banner during the protest near the official ceremony
Raphael Wong holds a banner during the protest near the official ceremony AFP / ISAAC LAWRENCE

In a small but significant departure from the past, the ceremony orders were given in Mandarin, rather than the Cantonese generally spoken in Hong Kong.

Lee delivered a speech praising China's imposition of a sweeping national security law in Hong Kong which came into effect just an hour before last year's handover anniversary.

"While safeguarding national security, residents continue to enjoy freedom of speech, freedom of press, freedom of assembly and demonstration and others according to the law," Lee said.

AFPTV / Katherine CHENG

Such assurances are repeatedly made by Hong Kong's leaders but reflect little of the city's current reality.

Over the last year, the national security law has criminalised a host of political views and transformed semi-autonomous Hong Kong.

Police and water cannon trucks patrol streets near the official ceremony
Police and water cannon trucks patrol streets near the official ceremony AFP / ISAAC LAWRENCE

Protest has been effectively outlawed, and many of the city's opposition leaders have been arrested or jailed, or have fled overseas.

Last month, the city's most outspoken pro-democracy newspaper Apple Daily shut down after its accounts were frozen under the national security law and multiple employees were arrested over articles the popular tabloid published.

Public gatherings of more than four people remain outlawed under anti-coronavirus measures, although the city currently has no active local outbreak.

On Wednesday, prominent human rights barrister Chow Hang-tung was arrested and charged with inciting others to join an illegal protest last month.

"This is persecution as well as privilege," she said in a message released via her lawyers. "Arresting me won't succeed in shutting up everyone else."

The only visible protest on Thursday morning was carried out by four activists from the League of Social Democrats, one of the few opposition parties still operating.

They held a banner near the official reception calling for the release of political prisoners.

Some 200 police officers followed their lonely march and stopped the group from getting too close to the exhibition centre where dignitaries were gathered.

"Police are acting as if they are facing some enormous enemy," Raphael Wong, one of the activists, told reporters.

"It prompts the question of whether Hong Kong has indeed fallen into a police state. But we will continue to voice out and defend our last space for free speech."

Police officers later cordoned off Victoria Park, ordering those playing sports as well as foreign domestic workers picnicking there to leave.

The force said it had seen online calls for people to attend "banned rallies".

Alexandra Wong, an elderly democracy activist better known as "Grandma Wong", was detained by multiple police officers a few hours later, while staging a single-person protest.

Other arresting offences announced by police included someone desecrating China's flag, another distributing "seditious" publications as well as someone possessing an offensive weapon and an imitation firearm.

After the flag-raising ceremony, civil service chief Patrick Nip was asked by a reporter whether he believed Hong Kong could now be described as a police state.

"I think that's an overstatement," he replied.