Riot police fanned out across a Hong Kong town on Tuesday to stamp out rallies marking the one-year anniversary of an attack against protesters by government supporters which sent anti-Beijing sentiment soaring.

Officers used pepper spray to disperse small groups of protesters and reporters inside a mall in the town of Yuen Long, near the border with China.

Hundreds were detained and searched throughout the night, and police said at least five arrests made, as officers used loudhailers to warn people against "unlawful assembly". 

At one point a police banner was raised saying crowds were in breach of a new security law Beijing imposed on the city after a man waved his own sign with the popular protest slogan: "Liberate Hong Kong. Revolution of our times."

Police said they issued 79 spot fines under emergency anti-coronavirus measures banning groups of more than four people. Pro-democracy activists and reporters wearing yellow press jackets were among those given tickets, AFP journalists saw. 

The attack inside Yuen Long station was a pivotal moment in last year's huge pro-democracy protests, compounding already swirling animosity towards the police and heralding a dramatic increase in political violence.

At least 40 people were injured when a group of stick-wielding men set upon protesters returning from a rally in the city. 

Videos of bloody beatings went viral, sparking accusations that police were too slow to respond and had allowed the attackers to gather and depart unmolested.

The force denies allegations of collusion and says 37 people have been arrested over the attack -- some with links to "triad" organised crime gangs.

Seven have been charged so far. 

In a statement on Tuesday, the police said they "understand the public's concern" and said the investigation had "high priority".

- 'Ugly side' -

More than 9,000 people have been arrested during pro-democracy protests over the last year, with some 1,500 charged. 

Local lawmaker Lam Cheuk-ting was one of those beaten in last year's attack. 

He was among those fined on Tuesday while trying to hold a small rally outside the station.

"I believe the police cannot face their own dark and ugly side in the July 21 attack, so they prevent us legislators from speaking out for the people," he told reporters afterwards. 

"But all Hong Kongers have seen it (the attack) clearly last year," he added.

Yuen Long is a blue-collar town in Hong Kong's rural New Territories bordering mainland China. It is a stronghold of staunchly pro-Beijing groups, as well as triad gangs.

The attack sparked huge protests by residents and left the community deeply divided.

"I feel helpless," a restaurant owner who gave just her first name Gigi told AFP on Tuesday. 

"Everyone knows what happened on that day, and yet the government refuses to face the truth."

As she was speaking, another man shouted that pro-democracy protesters were trying to start a "colour revolution" -- an accusation frequently levelled by Beijing.

Anger towards authoritarian China now permeates swathes of semi-autonomous Hong Kong. 

Last year's pro-democracy rallies raged for seven straight months, with violent clashes between police and protesters becoming routine.

In response, Beijing imposed a sweeping national security law on the city last month. China says it will restore stability and not hinder freedoms. 

But the law has already been used to criminalise some forms of peaceful protesting, such as advocating for independence or greater autonomy.

Similar national security laws on the mainland are used to crush criticism of China's leaders.