Smoke-choked Sydney ushered in the New Year with a huge fireworks display while police in Hong Kong fired tear gas at protesters, as crisis clouded some of the first celebrations to ring in the new decade.Billions around the world are set to cheer in 2020 after a tumultuous year dominated by demonstrations calling for political upheaval and action on climate change.Australia's largest city usually puts on a dazzling display of pyrotechnics over the glittering harbour to kick off global festivities, but this year's celebrations were overshadowed by calls to cancel the fireworks as devastating bushfires raged 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.But the show did go on in Sydney, where 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.Celebrations amid upheaval As the clock ticks past midnight, major cities in Europe, Africa and the Americas will join 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 tear gas and water cannon, as pro-democracy protesters took their movement into the new year with midnight countdown rallies and a massive march planned for January 1.Before midnight on Tuesday, thousands of protesters gathered across the financial hub, including along the waterfront of Victoria Harbour and at nightlife hotspot Lan Kwai Fong. Protesters at the harbourfront counted down chanting "Ten! Nine! Liberate Hong Kong, revolution now!" as they lit up their phones in a sea of lights.Smaller crowds of protesters in the Mong Kok district set fire to barricades - and riot police unleashed 2020's first volleys of tear gas in response.Earlier in the evening, thousands of people linked arms in human chains that stretched for miles along busy shopping streets and through local neighbourhoods.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.Bells, pop and politics 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 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.Looking to 2020 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.US President Donald Trump again dominated headlines in 2019, culminating in his historic impeachment by the Democratic-controlled House of Representatives on two counts of abuse of office and obstruction of Congress.The Republican-controlled Senate is unlikely to convict Trump in a trial expected to begin in January but the controversy over claims he pressured Ukraine to investigate Joe Biden, a rival in his 2020 re-election bid, will linger until the November poll.