At least eight people are dead and 11 more injured after a truck mowed down a crowd in lower Manhattan.
The vehicle drove the wrong way down the highway for several blocks, striking pedestrians on the way.
A suspect was shot by New York police after crashing the truck into another vehicle, reportedly a school bus carrying children, before exiting brandishing firearms.
The suspect is now in custody.
The incident took place near Battery Park City and the World Trade Centre memorial on Tuesday afternoon (local time).
Photos and videos are emerging on social media showing emergency services clustered at the scene near West St and Chambers St.
The FBI has taken charge of the investigation and officials say terrorism cannot be ruled out - although stress it is likely a "lone wolf" attack.
Live upates have now concluded. The following is an account of the attack as it unfolded.
6:00pm - Details emerge from inside the van involved in the crash
Reports have suggested that a note was left in the van, claiming the terror attack was committed in the name of ISIS.
12:17pm - 'This was an act of terror' - New York Mayor
New York Mayor Bill de Blasio says the attack "was an act of terror".
"A particularly cowardly act of terror, aimed at innocent civilians, aimed at people going about their lives who had no idea of what was about to hit them."
"New Yorkers will not be changed by an act of terror."
12:30pm - One victim from Belgium
One of the victims was from Belgium, their deputy Prime Minister Didier Reynders says.
12:10pm - World leaders react
Current and former world leaders and politicians are giving their support to the victims on social media.
11:52am - Suspect named
CBS News reports the suspect is Sayfullo Saipov, from Tampa, Florida, who came to the US in 2010 from Uzbekistan.
11:50am - New York City Halloween Parade kicks off
New York's annual Halloween Parade has kicked off, only blocks away from the site of the attack.
Officials have told people to go about their business as usual, but with extra vigilance.
11:45am - 'We must not allow ISIS to return' - Trump
Donald Trump has again taken to Twitter, this time to warn that the United States "must not allow ISIS to return, or enter… after defeating them in the Middle East and elsewhere".
11:40am - More footage emerges on social media
More footage is emerging of the suspect running around with fake guns after crashing his truck.
The original source is not clear.
11:25am - Suspect drove a rental van
Police say the suspect drove a rental van down the bike path for more than 20 blocks.
The truck was rented from retailer Home Depot.
11:15am - New York City Halloween Parade to go ahead
In less than two hours, New York's annual Halloween Parade will take place, only blocks away from today's attack.
The parade will take place in the West Village as planned.
Officials are stressing people should go about their lives as usual, but with vigilance.
11:00am - Donald Trump condemns attack
President Donald Trump has condemned the attack, calling the suspect "a very sick and deranged person".
"Law enforcement is following this closely. NOT IN THE U.S.A.!"
10:45am - Likely "lone wolf" attack
Police say the suspect is a 29-year-old male who is not from New York.
In a press conference, officers told reporters it was likely a "lone wolf" attack and there is no evidence suggesting a "wider plot".
A paintball gun and a pellet gun were found at the scene.
Police say they cann't rule out terrorism because of reports the suspect shouted "Allah Akbar" after exiting his truck.
Statements made by the suspect are "consistent" with a terrorist attack, NYPD Police Commissioner James O'Neill says.
New York will see an increased police presence in the coming weeks.
10:35am - Disturbing photos emerge
Photos show mangled bikes lying on the footpath after the truck driver rammed down pedestrians.
Photos also show the damaged truck, which reportedly crashed into a school bus carrying children before the driver was arrested.
10:25am - Death toll rises
New York City Mayor Bill de Blasio has now confirmed eight people died in the attack.
10:10am - Suspect deliberately crashed into bus carrying children, witnesses say
The suspect deliberately crashed into a bus carrying children before he was shot and arrested by police, NBC reports.
Witnesses describe seeing the driver swerve into the bus, before emerging brandishing firearms. He was then shot and taken into custody.
10:00am - Possible terror attack
The FBI has taken charge of the investigation, including the Joint Terrorism Task Force, and officials say they cannot rule out terrorism at this stage.
9:50am - White House responds
President Donald Trump has been briefed on the incident.
"Our thoughts and prayers are with all those affected," press secretary Sarah Sanders says.
9:40am - School in lockdown
Nearby, Stuyvesant High School has been placed on lockdown as a precaution.
9:30am - Suspect shot by police
Officials say the suspect was shot.
"The vehicle continued south striking another vehicle", New York police say on Twitter.
"The suspect exited the vehicle displaying imitation firearms & was shot by NYPD."
ABC News reports the truck hit a school bus.
The condition of the suspect is not known.
9:25am - Six reported dead
Six people are now reported dead and 15 more injured, according to NBC.
9:22am - More videos emerging of scene
A live Twitter feed shows the scene from above.
Multiple emergency services can be seen clustered at the scene.
9:15am: Witness describes moment shots were fired
A witness at the scene has described the moment he says a man started shooting.
"After the [truck] crash I ran up the bridge and when I ran up the bridge I saw a man with two guns," the man said on Twitter.
"All of a sudden four shots went off and we all just started running."
9:00am - Suspect arrested
A suspect has been taken into police custody, police say.
8:50am - photos and videos emerge on social media
Several photos and videos are emerging showing the panicked moment the van rammed into a crowd.