New Year's revellers defy terror alerts to welcome in 2017

People around the world shrugged off terror jitters to see in 2017 in style, as Sydney kicked off the party with a spectacular fireworks display that lit up its iconic harbour.

New York City was preparing for a veritable sea of humanity to converge on Times Square on Saturday for the annual descent of the New Year's Eve ball, a century-old tradition that will unfold this year under an unprecedented blanket of security.

As many as 2 million people, surrounded by a ring of 40-ton sand trucks and 7,000 police, are expected to gather in the "Crossroads of the World" to watch the glittering sphere complete its midnight drop, marking the beginning of 2017.

About 1.5 million people packed Australia's biggest city to watch as the midnight fireworks erupted from Sydney Harbour Bridge sending rainbow-coloured showers soaring into the night sky. 

Crowds in Hong Kong also flocked to the waterfront to watch fireworks explode over Victoria Harbour while in Japan thousands packed the streets of Tokyo to release balloons into the air.

The raucous celebrations drew to an end a year of bloodshed and misery that has seen the war in Syria, Europe's migrant crisis and numerous terror attacks dominate the headlines.

It has also been a year of political shocks, from Britain's vote to leave the European Union to the election of maverick leaders in the United States and the Philippines.

And the bloodshed continued on Saturday, with twin bomb blasts killing at least 27 in a busy market area in central Baghdad.

But this did not stop people flooding the streets of the Iraqi capital to celebrate and families in evening dress headed to swanky hotels for parties.

Fadhel al-Araji, a 21-year-old from the neighbourhood of Sadr City, already had his beer in the back of his car.

"Tonight is about fun... Everybody can do what they want and nobody cares. We need a night like this, Iraq needs it," he said, behind the wheel of his beat-up Toyota.

In the shattered Syrian city of Aleppo, 20-year-old student Abdel Wahab Qabbani was also determined to see in 2017 in a positive frame of mind.

"The last two years, I didn't go out for New Year. This time, I'm going to party," he said.

The Gulf emirate of Dubai marked the new year with its usual gigantic pyrotechnics off the world's highest skyscraper, Burj Khalifa, as well as other landmarks.

This year's celebration passed without problems, unlike last year when a fierce blaze broke out at a nearby tower.

Security was stepped up in major world cities from Sydney to New York as authorities fretted that the large crowds of people could present a target for extremists.

There were 2,000 extra officers in Sydney after a man was arrested for allegedly making online threats against the celebrations and garbage trucks were deployed to block any attempt to plough a vehicle into the crowd.

After a deadly attack on a Berlin Christmas market on 19 December, the German capital beefed up security, deploying extra police, some armed with machine guns.

"This year, what's new is that we will place concrete blocks and position heavy armoured vehicles at the entrances" to the zone around Brandenburg Gate, a police spokesman said.

However, visitors seemed undeterred by recent events as they began to gather under a freezing Berlin sky for a series of concerts and a large midnight fireworks display in the area.

In Paris, there were fireworks again, after muted 2015 celebrations following the massacre of 130 people by militants in the French capital.

Nearly 100,000 police, gendarmes and soldiers were deployed across France against the militant threat and President Francois Hollande inspected the security measures on the city's famous Champs Elysees.

Brussels, meanwhile, reinstated its firework show after last year's was cancelled at the last minute due to a terrorist threat.

With more than a million people expected to turn out to watch the ball drop in Times Square, New York is deploying 165 "blocker" trucks.

Even though city and federal officials say they are not aware of any credible threats, the specter of two deadly truck attacks in Europe looms over the Times Square rite of winter, which has attracted merrymakers since the early 20th century.

In devising the security plan, New York police officials say they heeded lessons learned from recent holiday attacks in Germany and France, where suspected militants intentionally plowed vehicles into crowds of pedestrians, killing dozens.

Despite the heavy police presence, or perhaps because of it, throngs of people, many from overseas, arrived hours early to get a prime view of festivities, which will include live musical performances by Mariah Carey, Thomas Rhett and Gloria Estefan. Although skies were cloudy, temperatures were expected to hold at a comfortable 40F (5C), with no rain in the forecast.

Extra security was also in place in Moscow, Istanbul and London.

Rome stationed armoured vehicles and greater numbers of security forces around the Coliseum and St Peter's Square, where Pope Francis will celebrate a "Te Deum" hymn of thanksgiving.

In a mass earlier on Saturday, the pontiff urged people to reflect on the plight of the young as the year drew to a close.

"We have created a culture that idolises youth... yet at the same time paradoxically we have condemned our young people to have no place in society," he said.

Elsewhere, issues other than terrorism threatened to dampen the party.

As many as two million people were expected at Rio's Copacabana beach. But with Brazil mired in its worst recession in a century, the fireworks have been cut to just 12 minutes.

And normally boisterous Bangkok was seeing in the new year on a more sombre note as the nation grieves for King Bhumibol Adulyadej, who died in October.

Nevertheless, revellers will at least get one extra second to enjoy the night's festivities.

At the stroke of midnight, there will be a "leap second" decreed by the International Earth Rotation and Reference Systems Service to allow astronomical time to catch up with atomic clocks that have called the hour since 1967.