Many who had come to watch spoke of overcoming their fears as New Yorkers proved their resilience in the aftermath of a car ramming that killed eight people and wounded 12 others in Manhattan near the 9/11 Memorial.
Dense crowds lined the route as the elite women began the race at 1420 GMT and the elite men followed a half-hour later in cool and cloudy conditions.
The city heavily bolstered security for the race, parking massive sand trucks to prevent vehicle attacks, stationing extra police on rooftops and deploying more anti-sniper units.
Hundreds of uniformed officers stood guard along the route, while plainclothes officers blended in with the crowds of spectators.
President Donald Trump insisted in an interview that aired Sunday, as he began an extended Asia trip, that Americans need never accept terrorism as inevitable.
"We cannot just say, 'Oh well, it's going to happen, let's get used to it.' We cannot allow it to happen," he said on the Full Measure syndicated television show. "I can tell you, the Trump administration is getting tougher and tougher and tougher."
But awareness of the potential threat was a constant in New York on Sunday. In iconic Central Park, where the race ends, a woman's amplified voice offered a repeated warning even before the race began: "Stay alert at all times."
'It did make me anxious'
Security in New York had already been boosted in 2013 after the Boston marathon attack that saw two youths of Chechen descent plant two bombs near the finish line, killing three people and wounding more than 250 others, including spectators.
As Sunday's marathon began, New Yorkers said they were coming together in defiance in the aftermath of the latest attack to strike America's most populous city.
In the Bay Ridge neighborhood of Brooklyn, Jean Schnell was waiting to see her daughter run past. Tuesday's attack made her nervous, she admitted, especially because she is from Boston.
"It did make me anxious about coming to New York, and her running, but we were going to come support her anyway, so it didn't really make any difference," she said. "I think today you have to live your life."
"What I do know, 100 percent, is that we're a very resilient nation and I don't think there are many tougher people than New Yorkers, and marathoners are pretty tough too," said American runner Shalane Flanagan, the 2008 Olympic silver medalist over 10,000 meters.
"So I think it's an opportunity to show resilience and strength and coming together... And when you come together as a community it really empowers people and helps people heal."
Flanagan said the carnage in New York had hit her hard as a veteran of the 2013 Boston marathon.
"It's obviously devastating and very concerning," she said of the New York attack.
"I've been in a terrorist attack in 2013 in Boston. I was there that day and had just completed my race. So it very much hits home and is very personal to me."
Flanagan said she may consider retiring if she stuns the field with a victory on Sunday.
Standing in her way is defending champion Mary Keitany of Kenya, who is chasing a fourth consecutive victory in New York.
In the men's race, Kenya's former world record holder Wilson Kipsang, who won the 2014 edition of the event, and last year's winner Ghirmay Ghebreslassie of Eritrea are among the favorites.
But a popular favorite was the American Meb Keflezhigi, an four-time Olympian born in Eritrea, who had announced, at age 42, that this year's race would be his last. He won the race in 2009.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)