Celebrations around the world to welcome in 2017 are being held amid heightened security in the wake of the deadly terror attacks in Berlin and Nice.

A number of Pacific islands - including Tonga and Samoa - were the first to countdown to the New Year at 10am Irish time.

Auckland in New Zealand welcomed the New Year at 11am Irish time, with a countdown at the city's Sky Tower followed by a firework and laser show. 

In Australia, which started celebrating the New Year at 1pm Irish time, an estimated 1.5 million people gathered at Sydney harbour to watch the fireworks spectacular.

An extra 2,000 police officers have been drafted in to the city while buses will be used to close off some pedestrian areas amid fears about a repeat of this year's extremist attacks in France and Germany.

Other parts of Australia will have to wait for another few hours until midnight and the start of 2017.

In Berlin, where 12 people were killed when a hijacked lorry was driven into a busy Christmas market, barriers have been installed around the landmark Brandenburg Gate to protect revellers.

Security has also been ramped up in Cologne in a move to prevent a repeat of last year's trouble, when police failed to failed to prevent a string of robberies and sexual assaults blamed largely on foreign men.

Hundreds of thousands are expected to attend a light show at the Champs-Élysées in Paris, with major road closures and traffic restrictions in place alongside the deployment of thousands of officers.

A soldier stands guard at the courtyard of the Louvre museum, in Paris, Friday, Dec. 30, 2016. Picture by Christophe Ena AP/Press Association Images

New Year's Eve fireworks and festivities are also going ahead in Brussels, after last year's celebrations were cancelled over fears of an attack.  

In New York's Time Square, where the famed glitter ball is due to descend at 5am UK time on 1 January, dozens of 20-ton refuse lorries weighted with an extra 15 tons of sand will block the streets around the celebrations, while there will be about 7,000 police officers on patrol.

The US security crackdown is not just confined to New York. In Las Vegas FBI and Secret Service agents are working alongside local police departments in order to keep safe more than 300,000 expected visitors for the extravagant celebrations.

In the Indian capital New Delhi and many other of the country's cities, security has been tightened around shopping centres and restaurants.