Police use pepper spray during clashes with protesters following an unsanctioned march through Hong Kong on Sunday. (AFP)

Fierce clashes broke out between protesters and riot police in Hong Kong Sunday as thousands marched through the strife-torn city, during a day of global protests aimed at casting a shadow over communist China’s upcoming 70th birthday.

Beijing is preparing for huge, tightly-choreographed festivities from Tuesday to mark the founding of the People’s Republic of China, including a massive military parade that will revel in the country’s emergence as a global superpower.

But ongoing unrest in Hong Kong threatens to upstage those celebrations as the semi-autonomous city boils with public anger over the erosion of its special freedoms under Beijing’s rule.

Democracy activists in the financial hub had vowed to ramp up their nearly four-month-long campaign ahead of Tuesday’s National Day celebrations, which Hong Kong protesters have dubbed a “Day of Grief”.

Sunday witnessed the most intense clashes in weeks as police used tear gas, rubber bullets and water cannons at multiple locations for hours during running battles with hardcore protesters hurling rocks and petrol bombs.

Using online forums and social media, demonstrators called for “anti-totalitarian” rallies to be held around the globe. Marches were held in Australia — where more than 1,000 people rallied in Sydney — and Taiwan, with more held in some 40 locations in Europe and North America.

Clashes broke out before Hong Kong’s unsanctioned march begun when angry groups in the Causeway Bay shopping district surrounded and heckled officers stopping and searching people.

But the tear gas police fired only emboldened the crowds, who then began walking through the streets in the thousands.

Some hardcore activists vandalized subway stations, tore down banners proclaiming the upcoming 70th anniversary celebrations and set fire to makeshift barricades.

Many marchers were holding so-called “Chinazi flags”, a version of the Chinese flag with yellow stars arranged in the shape of a swastika.

Police spent hours firing tear gas and rubber bullets at different downtown locations and made multiple arrests throughout the day.

An AFP reporter witnessed masked men repeatedly beating pro-democracy protesters in Wanchai district using long clubs.

It was unclear if the men were officers dressed as protesters, a tactic police increasingly use.

A news reporter was also treated by volunteer medics after being hit in the face with a police projectile, while at least two men were attacked by democracy demonstrators and a taxi was trashed.

One video posted by activists showed police spraying pro-democracy lawmaker Eddie Chu in the face point-blank with what was described as pepper spray, as he argues with the officers.

Hospital authorities late Sunday said 13 people were admitted, one in a serious condition.

A 20-year-old student, who gave his first name as Tony, was waving a Ukrainian flag.

Many of Hong Kong’s more hardcore protesters take inspiration from the 2014 Ukrainian Revolution, which ousted a pro-Russian president.

“We are hoping that if we connect to different parts of the world and fight against Chinese communism, we will win this movement,” he told AFP.

A 62-year-old protester, who gave his surname as Man, said he felt people’s livelihoods and freedoms had deteriorated since Britain handed Hong Kong to China in 1997.

“I don’t want to topple the regime, but it definitely needs to change,” he told AFP. “It needs to seek a reconciliation with the people.”

A protestor holds a portrait of China President Xi Jinping as demonstrations are held infront of the China embassy in Warsaw. (AFP)

Over the last 17 weeks, Hong Kong has witnessed the worst political unrest since its handover to China, with huge pro-democracy rallies as well as increasingly intense clashes between police and a minority of violent protesters.

The city’s summer of discontent was triggered by an extradition bill to the mainland that has now been shelved.

But the movement has since morphed into a call for free elections and less intervention from Beijing.

On Friday and Saturday night, tens of thousands of people turned out for two peaceful rallies.

There were brief clashes late Saturday when police used water cannon and tear gas to beat back small groups of protesters hurling bricks and petrol bombs at a government building.

Students are planning a one-day strike on Monday while activists have called for people to dress in black on Tuesday.

Under the policy of “one country, two systems”, China has offered tiny Hong Kong certain liberties denied to citizens on the mainland — including freedom of expression, unfettered access to the internet, and an independent judiciary.

But the arrangement is due to expire in 2047, and many accuse Beijing of already eroding some of those freedoms.

Protesters are calling for an independent inquiry into alleged police abuses, an amnesty for those arrested, and the right to elect their leaders, demands dismissed by China and city leader Carrie Lam.