Protesters in Hong Kong wave "Free Hong Kong" placards ahead of China’s National Day

Thousands defy police tear gas rounds on Sunday to hold an unsanctioned march through the city

Hong Kong: Thousands of Hong Kongers defied police tear gas rounds on Sunday to hold an unsanctioned march through the city, part of a coordinated 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 marking seven decades since the People's Republic of China was founded, including a huge 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 called for “anti-totalitarian” protests to be held around the globe on Sunday.

Marches were held in Australia and Taiwan, with more planned in Europe and North America later in the day.

Online forums used to coordinate the deliberately leaderless protest movement called for protesters to begin the Hong Kong march in Causeway Bay, a busy shopping district filled with luxury malls and fashion retailers.

Police fired tear gas after angry crowds in the district surrounded and heckled officers who had conducted stop and searches ahead of the march.

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

Some hardcore activists vandalised subway stations and tore down banners proclaiming the upcoming 70th anniversary celebrations.

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

Later in the afternoon police fired tear gas and rubber bullets at different locations across the city centre.

Many of Hong Kong's more hardcore protesters have taken inspiration from the 2014 Ukrainian Revolution which ousted a pro-Russian president.

Over the last 17 weeks, Hong Kong has witnessed the worst political unrest since its handover to China in 1997 with huge pro-democracy rallies as well as increasingly intense clashes between police and a minority of violent protesters.