Riot police and protesters in Hong Kong fought brief skirmishes near the Chinese border on Saturday, the latest clashes during huge pro-democracy protests that have battered the financial hub for more than three months.

Police fired tear gas and rubber bullets at small groups of hardcore activists who had built barricades in the outlying town of Tuen Mun, some of them lobbing bricks and at least one Molotov cocktail.

Multiple people were seen being arrested in the clashes, which were less sustained than the intense battles of previous weekends.

Protesters highlighted viral video of police late Saturday in Yuen Long town using pepper spray to subdue a 73-year-old man known as "Uncle Chan" or "Grandpa Chan," described as an prominent activist who urged riot police to show restraint during demonstrations.

Hong Kong has been convulsed by 16 straight weekends of sometimes violent rallies calling for greater democratic freedoms and police accountability.

The movement is the biggest challenge to China's rule since Hong Kong was handed back by Britain in 1997 and shows no sign of ending, with city leaders and Beijing taking a hard line.

In a now familiar pattern, the day began with a peaceful rally.

At one point, a handful of protesters pulled down China's flag flying outside a local government office and burned it.

Tensions spiked after police snatch squads rushed into a park where crowds had gathered and made a series of arrests.

Hundreds of hardcore activists then built barricades and dismantled nearby fences to arm themselves with makeshift clubs. Objects were also thrown onto nearby train tracks.

But protesters showed little appetite in holding ground, quickly retreating as soon as police advanced.

By Saturday evening, pockets of demonstrators and police were playing a familiar game of cat and mouse.

There were also skirmishes on Saturday evening in Yuen Long, a nearby town where protesters were attacked by local Beijing supporters in July.

People had gathered to mark the two month anniversary of that assault -- further fuelling animosity towards police, who were accused of being too slow to respond.

Officers were seen making multiple arrests on Saturday evening in Yuen Long, sparking angry chants from locals and protesters.

Calvin Tan, 22, was among those taking part in the rally earlier in the day.

He said most protesters were prepared for a "long-term fight".

"Every small protest matters, even though it doesn't seem to help that much, it's like each small step in a marathon," he told AFP.

'No choice'

Tan described himself as a moderate who avoids clashes with police. But he said he ideologically supports those on the frontlines.

"When facing institutional violence, we have no choice but to respond with street fights," he said. "We have tried peaceful, rational and non-violent methods and continued using these ways to fight for our demands."

Hong Kong has been plunged into crisis this summer.

The protests were ignited by a now scrapped plan to allow extraditions to the authoritarian Chinese mainland.

But when local leaders and Beijing refused to budge it snowballed into a wider campaign for democracy, fuelled by animosity towards the police.

Protesters have hurled rocks, bottles and petrol bombs as well as used slingshots in their battles with officers, who have responded with tear gas, rubber bullets and water cannons with growing frequency.

On Friday Amnesty International released a report accusing Hong Kong's police of using excessive force, in some cases amounting to torture.

The UN's rights watchdog has also criticised the city's law enforcement.

But police have rejected those condemnations, saying their officers have used proportionate force against hardcore protesters who are showing increasing levels of violence.

"Our officers are worried that... violence has got to such a level that they might have to kill someone or be killed themselves," he said, speaking on condition of anonymity.

"We have been so restrained but in the face of such violence, this pressure has become extremely dangerous."