Europe ushered in the New Year amid heightened security fears as German police evacuated two train stations in Munich, citing a tip about a planned militant attack, and Belgium arrested three people over an alleged plot.
Security forces in many capitals were on raised alert after a year of militant attacks, the biggest of which killed 130 people in Paris in November and was claimed by Islamic State.
Soldiers were on the streets of Paris, and police forces in London, Madrid, Berlin and Istanbul increased their presence as Europeans turned out to celebrate the arrival of 2016.
In New York, police tightened security for the traditional New Year's Eve dropping of the crystal ball in Times Square, where more than a million people were expected.
Celebrations in Germany took on a sombre note when police warned of a possible terror attack and evacuated two train stations in Munich.
A police spokeswoman said a search for possible suspects had been launched and a tweet from German police said "we take the threat very seriously".
In Belgium, police were holding three people for questioning on Thursday as part of an investigation into an alleged plot to carry out an attack in Brussels on New Year's Eve.
Authorities on Wednesday called off the usual New Year's Eve fireworks display in the capital, citing fears of a possible militant attack.
Police had initially detained six people after searches at six locations in the Belgian capital and one just outside the city. They also seized computers, mobile phones and equipment for airsoft, a sport using guns that shoot non-lethal plastic pellets.
Three of the six were released late on Thursday. Prosecutors said they were holding the other three for a further 24 hours.
French President Francois Hollande said on Thursday the country was "not finished with terrorism" and used a New Year's message to defend controversial plans to strip citizenship from those convicted of terrorism offences.
"The threat is still there," he said in a televised address. "It remains in fact at its highest level, and we are regularly disrupting planned attacks."
In Istanbul, police said they had ramped up the number of officers on the streets by around 10,000.
Turkish police on Wednesday detained two suspected IS members they believe to have been plotting New Year's Eve suicide attacks in the capital Ankara, where less than three months ago a double suicide bombing killed more than 100 people.
While many people turned out for public New Year celebrations, the year of militant attacks produced more of a mood of worry and uncertainty for others.
"Normally, I go out to the bars on the Asian side of Istanbul," said 26-year-old IT expert Seyda Yilmaz. "But this year, because of the danger of ISIS (Islamic State), I will spend New Year's Eve in my home."
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