A small child has been pictured grimacing in agony after he was hit by tear gas thrown by police to disperse protesters in Hong Kong.

The little boy's pain was captured during the massive wave of protests in the special territory.

The heartbreaking incident happened during a rally against police brutality following months of mass demonstrations.

Hong Kong's protests were initially triggered over a now-withdrawn extradition bill with China, but have since transformed into a pro-democracy movement.

Protesters set fire to shops and hurled petrol bombs on Sunday after riot police fired tear gas, water cannons and rubber bullets to disperse the thousands rallying in one of Hong Kong's key tourism districts, Reuters reports.

The boy screws up his face in pain (
Image:
LYNN BO BO/EPA-EFE/REX)
Bystanders aid the child who is in visible pain after being affected by police tear gas (
Image:
LYNN BO BO/EPA-EFE/REX)

The protesters clashed with police in Hong Kong's Tsim Sha Tsui harbour-front hotel district late into the evening.

Police used a water cannon, tear gas and pepper spray, and claim protesters later returned fire with petrol bombs and set fire to shops, Reuters reports.

The protesters, many in all-black clothing and face-masks now banned under a resurrected British colonial-era law, had gathered to denounce perceived police brutality during more than four months of often-violent unrest in the city.

It was the 21st straight weekend of protests, marked by a notable rise in the jeering and verbal abuse of police but not in the scale of violence.

Police said one man was beaten up by "masked rioters".

A young boy is given a gas mask after police fired tear gas to disperse pro-democracy protesters in Tsim Sha Tsui (
Image:
AFP via Getty Images)
Pro-democracy protesters throw tear gas canisters back toward police during a rally against brutality (
Image:
LYNN BO BO/EPA-EFE/REX)

"The police warn all rioters to stop all illegal acts immediately," they said in a statement.

Cat-and-mouse chases continued into Sunday night, with protesters running away from streams of blue-dyed water from a police cannon and tear gas, only to re-appear seconds or minutes later.

Protesters shouted obscenities in colourful Cantonese at "black police", referring to their perceived over-use of force.

"Fight for Hong Kong!" protesters shouted, "five demands, not one less", a reference to demands for universal suffrage and an independent inquiry into police actions, among other things.

The rallying cry of the protesters was also for the protection of "Muslims, journalists and the people".

A man reacts to tear gas as he sits inside a shop in Hong Kong's tourism district of Tsim Sha Tsui (
Image:
REUTERS)
A riot police officer fires a tear gas canister toward anti-government demonstrators on Sunday (
Image:
REUTERS)

Protesters are angry about what they view as creeping Chinese interference in Hong Kong.  

The huge demonstrations sweeping the country mark the largest scale of political unrest since the former British colony was returned to Chinese rule in 1997.

It was handed back to China under a "one country, two systems" formula intended to guarantee freedoms not seen on the mainland.

The protests achieved an early win with a back down from Hong Kong territory leader Carrie Lam agreeing to withdraw a controversial extradition bill with China.

Tear gas covers a road during the rally (
Image:
LYNN BO BO/EPA-EFE/REX)

The protests have continued, and have led to accusations of police violence, peaking in a huge demonstration that saw hundreds of thousands hit the streets across the territory during China's Communist Party's 70th anniversary celebrations just weeks ago.

Footage showing police shooting a teenage boy at close range during the anniversary day rallies came as more than 50 protesters were injured in Hong Kong on that  day, with the youngest casualty claimed to be just 11 years old.

Police deny accusations of brutality, saying they have shown restraint amid the wave of demonstrations.

China has continued to deny meddling in Hong Kong, and has accused foreign governments, including the United States and the UK, of stirring up trouble.