Smoke-choked Sydney ushered in the New Year with a huge fireworks display, kicking off celebrations for billions around the world and ringing in the new decade.

Australia's largest city usually puts on a dazzling display of pyrotechnics over the glittering harbour, but this year's celebrations were overshadowed by calls to cancel the fireworks as devastating bushfires rage across the country.

Toxic smoke haze has shrouded Sydney for weeks and a petition to cancel the event out of respect for fire victims attracted more than 280,000 signatures.

Fireworks displays were scrapped in Australia's capital, Canberra, and Sydney's western suburbs due to elevated fire danger and extreme weather conditions.

Critics wanted Sydney to use the Aus$6.5 million ($4.5 million) spent on the display to fight bushfires ringing the city, but officials say the event is worth Aus$130 million to the economy and cancelling it would not help those impacted by the fires.

"We have committed to harnessing the enormous power of the event to raise more money for drought- and fire-affected communities," Sydney Lord Mayor Clover Moore said.

More than 100,000 fireworks lit up the skyline for the hundreds of thousands of spectators thronging the city centre.

Crowds were warned to take care as strong winds gusted in the harbour, forcing the cancellation of a boat display that would have blasted water into the sky.

As the clock ticks past midnight, major cities in Asia, Europe, Africa and the Americas will embrace the celebrations, but in many places the festivities will be marked by turmoil and political upheaval.

After more than six months of near-daily demonstrations, Hong Kong ushered in 2020 with a series of pro-democracy rallies planned for New Year's Eve.

Protesters formed human chains that stretched across busy shopping streets and through residential neighbourhoods.

As the clock struck midnight in North Korea, a large crowd gathered for a concert in the centre of Pyongyang cheered as the clock struck midnight, with fireworks bursting in the sky above a neon-lit stage hosting a tightly-choreographed dance performance.

Across the border, South Koreans flocked to bell-ringing ceremonies - a traditional ritual marking the start of the new year - with thousands watching the ritual in central Seoul alongside performances by Korean pop stars.

In Paris, 250,000 to 300,000 people usually gather on the Champs-Elysees to welcome the New Year, but turnout could suffer amid a gruelling transport strike that has spelt weeks of misery for commuters.

Midnight in London will be marked by the chimes of Big Ben, which has been silent during a long restoration, as traditional fireworks are set off over the Thames for the last new year before Brexit.

It follows a year of political wrangling that led to the resignation of Prime Minister Theresa May and culminated in Conservative Prime Minister Boris Johnson pledging to leave the European Union on January 31.

In Moscow, President Vladimir Putin is set to deliver his annual New Year address, 20 years after he was elevated to the presidency by Boris Yeltsin's shock resignation in his 1999 end-of-year speech.

Russia will celebrate the new decade over several time zones, with Muscovites flocking to the centre of the capital for fireworks over the Kremlin.

As partygoers embrace the festivities, attention will turn to 2020 and whether it will be as tumultuous as the previous year, which saw an explosion of demonstrations as people demanded an overhaul of entrenched political systems and action on climate change.

The protests in Hong Kong, sparked by a now-abandoned bill to allow extraditions to the authoritarian mainland, have since morphed into a popular revolt against Beijing's control.

The city's biggest crisis since the former British colony's return to Chinese rule in 1997 appears set to spill over in 2020, with protesters already preparing the year's protest schedule and the government showing no sign of backing down.

Anti-government protests also swept Latin America, North Africa and the Middle East in 2019, including mass demonstrations that brought down leaders in Lebanon, Algeria, Sudan and Bolivia. Climate change sparked rallies worldwide calling for action, initiated by Swedish teen activist Greta Thunberg, as temperatures soared above records, Iceland lost its first glacier to climate change, and Venice was swamped by flooding not seen in decades.