Some two million people braved New York's coldest New Year weather for a century to welcome 2018 in Times Square, with police enforcing intensely strict security.

Residents and tourists faced the worst New Year freeze since 1917, with temperatures of nine degrees Fahrenheit (minus 12.7 Celsius).After two attacks in the city inspired by the Islamic State group, in December and October, security was tightened with more than 20 roads closed. Barricades and sand-filled trucks were in place, along with bomb-sniffing dogs and more officers on the ground.

Police also carried out two rounds of security screening on everyone who entered the Times Square security perimeter, the largest in history.

On entering the secured area, revelers emptied and discarded their purses and other small bags, which were not allowed.

After a mass shooter in October killed 58 people in Las Vegas by firing into a crowd from his hotel room, police also deployed personnel to hotels and restaurants for the first time.

"This is like the most exciting day in my life. I am very committed (to facing the cold)," said Hader Ghulam, 27, a Pakistani student based in Oklahoma visiting the city for the first time.

He told AFP he was impressed by the security measures and felt "pretty safe."

Amber Gerrits, a student from Michigan, said she was not worried.

"I think it'll be all right," she said, adding that "a lot of layers" of clothing helped prepare for temperatures which the National Weather Service warned were "dangerously cold."

"And we plan on not drinking too much water," the 21-year-old said, referring to the difficulties of finding a public bathroom among the huge crowds.

The celebrations in Times Square featured music performances, fireworks, 3,000 pounds (1,360 kilograms) of confetti and 25,000 balloons.

Mayor Bill de Blasio and this year's special guest, activist Tarana Burke -- leader of the #MeToo movement against sexual harassment -- pressed a button at midnight to start the traditional dropping of a large luminous ball down a pole.

Among the celebrities joining the party were Mariah Carey -- whose performance last year was plagued with technical problems -- Neil Diamond, Nick Jonas and Camila Cabello.