The world rang in the new year on Wednesday with spectacular firework displays from Sydney to Tokyo, though celebrations in Australia were overshadowed by deadly wildfires and the festive mood in Hong Kong and India was dampened by protests.
Around a million revellers thronged Sydney harbour and nearby districts to watch more than 100,000 fireworks explode above the city, even as thousands of people along Australia's eastern seaboard sought refuge from the bushfires on beaches.
Hong Kong cancelled its popular New Year's Eve fireworks in Victoria Harbour due to security concerns as protesters formed giant human chains and marched through shopping malls, vowing to continue to fight for democracy in 2020.
Thousands of Indians also planned to greet the new year with protests, angered by a citizenship law that they say will discriminate against Muslims and chip away at India's secular constitution.
Sydney decided to press ahead with its fireworks display despite calls by some members of the public for it to be cancelled in solidarity with fire-hit areas in New South Wales, of which the city is the capital.
Sydney mayor Clover Moore said planning had begun 15 months ago and that the event also gave a boost to the economy.
Some other towns in eastern Australia cancelled their new year celebrations as naval vessels and military helicopters helped firefighters to rescue people fleeing the fires, which have turned swathes of New South Wales into a raging furnace.
The fires have killed at least 11 people since October, two of them overnight into Tuesday, destroyed more than 4 million hectares (10 million acres) and left many towns and rural areas without electricity or mobile coverage.
Some tourists trapped in Australia's coastal towns posted images of blood-red, smoke-filled skies on social media. One beachfront photograph showed people lying shoulder-to-shoulder on the sand, some wearing gas masks.
Elsewhere, revellers from Auckland in New Zealand to Pyongyang, capital of isolated North Korea, welcomed the new year with firework displays. In Japan, people took turns to strike Buddhist temple bells, in accordance with tradition.
In Hong Kong, rocked by months of sometimes violent pro-democracy demonstrations, protesters were urged to wear masks at a New Year rally called "Don't forget 2019 - Persist in 2020", according to social media posts.
A "Symphony of Lights" was planned instead of the firework display, involving projections on the city's tallest skyscrapers after a countdown to midnight.
"This year there are no fireworks, but there will probably be tear gas somewhere," said 25-year-old IT worker Sam. "For us it’s not really New Year’s Eve. We have to resist every day."
Some 6,000 police were deployed and Chief Executive Carrie Lam appealed for calm and reconciliation in her New Year's Eve video message.
The protests began in June in response to a now-withdrawn bill that would have allowed extraditions to mainland China, where courts are controlled by the Communist Party, and have evolved into a broader pro-democracy movement.
In India, protesters angry about Prime Minister Narendra Modi's new citizenship law planned demonstrations on Tuesday evening in the capital New Delhi, in the grip of its second coldest winter in more than a century, as well as the financial hub Mumbai and other cities.
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.
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.