New York City police officer Ryan Nash was responding to a call about an emotionally disturbed person at a high school not far from the World Trade Center when someone reported an accident on the bike path outside.

Nash and his partner, John Hasiotis, raced to a gruesome sight: A man in a truck had slammed into a school bus after mowing down people in a bike lane. He was waving guns around and yelling. Nash, 28, told him to drop the weapons and then fired once, striking the man.

Nash stopped the attacker, Sayfullo Saipov, in his tracks, but the officer is too modest to admit he's a hero, officials said.

"He was a hero," said Gov. Andre Cuomo. "And the NYPD is not just the leadership, it's the men and women who are out there every day who are on the first line, and I think officer Nash really showed how important they are and how talented and how brave."

Saipov was actually wielding a pellet gun and a paintball gun, authorities said, but they looked like real guns. Witnesses reported the 29-year-old hollered "Allahu akbar," or "God is great!" in Arabic.

Hasiotis and two other officers, Michael Welsome and Kevin McGinn, secured the area, took witness statements and grabbed the guns. And they showed restraint by not firing their weapons into the crowded area, police said.

"While we mourn the terrible loss of life and the injuries to innocent people, we are proud of, and grateful for, the quick action of a team of police officers who responded to cries for help and took charge of a chaotic and dangerous situation," said police union president Patrick Lynch.

Mayor Bill de Blasio and other civic leaders commended Nash for his cool head -- and said he was a humble officer who felt like he was just doing his job.

"What he did was extraordinary. It gave people such faith and such appreciation in our police force," de Blasio said.

Nash was taken to the hospital for a ringing in the ears and met with Police Commissioner James O'Neill.

"In a typical fashion of an NYPD cop, he thinks what he did was not an act of heroism," O'Neill said. "He thinks it's ... why he joined the police department."

No one answered the door at Nash's two-story yellow Cape Cod-style home on Long Island on Wednesday. Neighbors said he moved in sometime in the past year and largely kept to himself, though he often jogged outside. It's not clear if he could talk publicly because he'd be expected to be a witness in the expected criminal proceeding.

"What he did was amazing, you know, helping to keep anybody else from being killed," said neighbour Dino Cortina. "It's just nice to know, nice guy."

Cortina's wife, Margie, said Nash kept to himself but was friendly.

"We feel safer now knowing that there's a hero, you know, in our neighbourhood."

In Tuesday's attack, Saipov drove his speeding truck for nearly a mile along the bike path, running down cyclists and pedestrians, then crashed into a school bus, authorities said. Eight people were killed and 12 injured.

"All New Yorkers should be thankful to Ryan and his partner. They showed great courage," O'Neill said.