"Blocking trucks" and rooftop snipers have been deployed across New York amid increased security at the city's annual marathon.
The 26.2 mile road-race – which attracts over 50,000 runners and 2.5 million spectators – is going ahead just five days after the deadly truck attack in the city killed eight.
New York police have deployed rooftop snipers, heavy weapons, helicopters and sand-filled trucks to protect against vehicle attacks.
Uniformed police officers, counter terrorism officers and police dogs will also patrol the event.
New York Governor Andrew Cuomo said in a statement: "While there is no apparent threat, we are ramping up security across New York City with patrols and resources from various state law enforcement agencies out of an abundance of caution."
Mr Cuomo also confirmed that extra officers will be deployed at airports, bridges, tunnels, and mass transit systems both before and after the race.
Last week's vehicle attack on a Manhattan bike path by Uzbek immigrant Sayfullo Saipov was New York's worst since 9/11.
The city's Halloween parade – which routinely attracts around 60,000 participants – went ahead with increased security measures just hours after the attack.
Now runners – who train for months and wait for years to win a spot in the fiercely competitive draw for places – are equally determined not to be cowed by terror.
With staggered start times, kicking off with wheelchair racers at 8.30am US time (1.30pm UK time) followed by professional runners and then amateur runners, most of the participants were expected to finish the race by around 4pm US time (9pm UK time).
"There's been no spike in cancellations that we've seen," said Chris Weiller, a spokesman for race organizer the New York Road Runners.
The race – now in its 47th year – been cancelled only once, following hurricane Sandy which caused severe damage in the city and killed over 200 people across the country in 2012.
This is not the first time US runners have had to worry about threats to their safety.
In 2013, three people were killed and more than 260 were injured when two brothers inspired by al Qaeda set off two bombs at the finish line of the Boston Marathon.
The NY Marathon route connects all five of the city's boroughs, with runners starting near the Verrazano-Narrows Bridge in Staten Island, winding through Brooklyn, Queens and Manhattan, before looping up to the Bronx and back to Manhattan.