Police fired tear gas during a demonstration in the Wanchai district in Hong Kong. (AFP)

Hong Kong police fired tear gas at pro-democracy protesters on Sunday after tens of thousands hit the streets once more to defy a ban on face masks despite half the city's subway stations remaining closed.

Large crowds marched through torrential rain in unsanctioned rallies on both sides of Victoria Harbour while police battled protesters in multiple locations, plunging the finance hub into chaos once more.

A taxi driver was beaten bloody in the district of Sham Shui Po after he drove into a crowd that had surrounded his car.

"Two girls were hit by the car and one girl was trapped between the car and a shop," an eyewitness, who gave his surname Wong, told AFP, adding the crowd managed to push the car off the wounded woman.

An AFP photographer on scene saw volunteer medics treating both the driver and the two women before paramedics and police arrived. Protesters had smashed up the taxi.

Shortly before a crowd ransacked nearby government offices.

Activists have staged unsanctioned flashmob rallies across the strife-torn city in recent days -- some vandalising subway stations and shops -- after Hong Kong's leader outlawed face coverings at protests, invoking colonial-era emergency powers not used for half a century.

Pro-democracy lawmakers went to the High Court Sunday morning seeking an emergency injunction against the ban, arguing the emergency powers bypassed the legislature and contravened the city's mini-constitution.

But a senior judge dismissed their case.

The law allows the city's unelected pro-Beijing leader Carrie Lam to make "any regulations whatsoever" during a time of public danger.

She used it to ban masks -- which protesters have used to hide their identity or protect themselves from tear gas -- and warned she would use the powers to make new regulations if the unrest did not abate.

The ban was welcomed by government supporters and Beijing.

But opponents and protesters saw it as the start of a slippery slope tipping the international finance hub into authoritarianism.

And it has done little to calm tensions or stop crowds coming out.

"If Carrie Lam wants to de-escalate the situation, this is not the right way," a 19-year-old protester who gave his first name Corey, told AFP as he marched under a forest of umbrellas on the main island.

Two teen protesters shot

Hong Kong has been battered by 18 consecutive weekends of unrest, with widespread public anger over Chinese rule and the police response to protests.

The rallies were ignited by a now-scrapped plan to allow extraditions to the mainland, which fuelled fears of an erosion of liberties promised under the 50-year "one country, two systems" model China agreed to ahead of the 1997 handover by Britain.

After Beijing and local leaders took a hard line, the demonstrations snowballed into a wider movement calling for more democratic freedoms and police accountability.

Protesters burn debris in the street during a demonstration in Wanchai district in Hong Kong. (AFP)

Lam has refused major concessions but struggled to come up with any political solution, leaving police and demonstrators to fight increasingly violent battles as the city tips into recession.

The worst clashes to date erupted on Tuesday as China celebrated 70 years of Communist Party rule, with a teenager shot and wounded by police as he attacked an officer.

A 14-year-old boy was then shot and wounded Friday night when a plainclothes police officer, who was surrounded by a mob of protesters throwing petrol bombs, fired his sidearm.

That night, masked protesters went on a rampage in dozens of locations, trashing subway stations and businesses with mainland China ties.

Much of subway shuttered

The city's subway system -- which carries four million people daily -- was shut down entirely Friday night and throughout Saturday, bringing much of the metropolis to a halt.

Major supermarket chains and malls announced they were closing, leading to long lines and panic buying.

Thousands of masked protesters still came out onto the streets throughout Saturday despite the mask ban and transport gridlock, although the crowds were smaller than recent rallies.

On Sunday, the subway operator said 45 stations would open but 48 remained shuttered, many of them in the heart of the city's main tourist districts as well as those areas hit hardest by the protests and vandalism.

Lam has defended her use of the emergency powers and said that she is willing to issue more executive orders if the violence continues.

"We cannot allow rioters any more to destroy our treasured Hong Kong," Lam said in a stony-faced video statement on Saturday.

But opposition lawmakers said the use of such a law had deepened the crisis.

"I would say this is one of the most important constitutional cases in the history of Hong Kong," lawmaker Dennis Kwok told reporters before Sunday's ruling.

Protester demands include an independent inquiry into the police, an amnesty for the more than 2,000 people arrested and universal suffrage -- all requests rejected by Lam and Beijing.