At least eight people have been killed and 11 seriously hurt in New York after the driver of a truck mowed down people on a cycle path in Lower Manhattan.
A 29-year-old man who emerged from the white pick-up truck was shot by a police officer and arrested.
US media named him as Sayfullo Saipov, an immigrant who came to the US in 2010 and had apparently settled in Florida.
New York City Mayor Bill de Blasio said it was "cowardly act of terror aimed at innocent civilians".
He added: "We know that this action was intended to break our spirit. But we also know that New Yorkers are strong, New Yorkers are resilient and our spirit will never be moved by an act of violence and an act meant to intimidate us."
President Donald Trump tweeted: "My thoughts, condolences and prayers to the victims and families of the New York City terrorist attack. God and your country are with you!"
New York Police Department (NYPD) Commissioner James O'Neill said the injured had "serious but non-life threatening injuries".
He described what had occurred, based on the latest information he had received:
- Shortly after 15:00 local time (19:00 GMT), a vehicle rented from retailer Home Depot struck cyclists and pedestrians while being driven along the West St-Houston St path for many blocks
- The vehicle then hit a school bus, injuring two adults and two children on board, and came to a halt
- The driver emerged holding what appeared to be two handguns and made a statement "consistent with a terror attack"
- He was shot in the abdomen by a police officer stationed near the scene
- A paintball gun and a pellet gun were recovered from the scene
"The dead and injured were just going about their days, heading home from work or from school or enjoying the afternoon sun on their bicycles," the commissioner said.
"This is a tragedy of the greatest magnitude for many people, for many families here in New York and beyond today."
Mangled bicycles littered the scene of the attack, which occurred as much of the city was celebrating Halloween.
One witness, identified as Eugene, told ABC Channel 7 that he saw the white pick-up truck driving fast down the cycle path alongside the West Side Highway, near Stuyvesant High School, at full speed and hitting a number of people.
He also reported hearing about nine or 10 shots.
Another witness, who gave his name as Frank, told local TV network NY1 that he had seen a man running around an intersection and heard five to six gunshots.
"I saw he had something in his hand, but I couldn't tell what it was. But they said that it was a gun...
"When the cops shot him, everybody started running away and it got a little bit crazy right there. So when I tried to look again, the guy was already down."
President Donald Trump had been briefed on the incident, the White House said.
In separate tweets, he said:
- "In NYC, looks like another attack by a very sick and deranged person. Law enforcement is following this closely. NOT IN THE U.S.A.!"
- "We must not allow ISIS [Islamic State] to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"
Piecing the evidence together
By Nick Bryant, BBC News, New York
At the moment the New York authorities are saying that was a lone wolf, that it wasn't part of a wide conspiracy or plot.
But this is an active crime scene at the moment and they are still trying to piece together precisely what happened.
The attack happened on Halloween, one of the most festive days in the New York calendar.
The pavements were crowded with kids in costumes and there are still children trick-or-treating just yards away - it's a bizarre scene.
But it shows how New York absorbs this kind of thing.
We are just yards from Ground Zero, a site which reminds all New Yorkers of that awful day back in 2001. It didn't take police long to confirm that the city had once again been the target of terror.