NEW YORK, United States (AFP) — Five days after the worst attack on New York since September 11, 2001, the city was staging a show of defiance Sunday, with some 2.5 million people packing the streets to cheer on 50,000 marathon participants from around the world.

The day after an Uzbek man killed eight people and wounded 12 others in Manhattan near the 9/11 Memorial, Mayor Bill de Blasio had vowed that the race would go on, adding that New Yorkers would “not be cowed.”

On Sunday, he told ABC that “this event is life-affirming every year, but this year (it is) even more powerful.”

Indeed, crowds heavily 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 more sand trucks to prevent vehicle attacks, stationing extra police on rooftops and deploying more anti-sniper units.

In addition to uniformed officers along the route, plainclothes officers were blending in with the spectators.

President Donald Trump insisted in an interview that aired Sunday that the United States would 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.”

Security had already been boosted in 2013 after the Boston marathon attack that saw two youths of Chechen descent plant two bombs near the arrival line, killing three people and wounding more than 250 others, including spectators.

The Islamic State group has described Sayfullo Saipov — the 29-year-old man charged with driving a rented pickup truck down a bike path in Manhattan as children and their parents prepared for Halloween — as one of its own, prompting President Donald Trump to call for his execution.

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.

The race that takes runners through all five boroughs and culminates at Manhattan’s iconic Central Park will be a show of “resilience” for a city recovering from tragedy.

“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 10,000m.

“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 having been a part of a terrorist attack.”

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 this time around.