In the footage, a man appearing to be the attacker runs through traffic in downtown Manhattan.
The man appears to be holding a firearm in his hand as he swerves past cars to make his escape.
US media have named the attacker as Sayfullo Saipov, a Florida resident who came to the US in 2010.
It is believed he was armed with a paintball gun and pellets, but authorities have refused to give out further information.
“Many” people are thought to have been injured in the attack, with up to 11 thought needing medical attention.
New York City Police confirmed the truck, which appeared to be a Home Depot rental van, drove onto a pedestrian and cycle path alongside the West Side Highway, hitting several people.
The truck then hit a school bus, which was carrying two adults and two children.
The driver entered the bike path in lower Manhattan at 3.05pm, New York City Police Commissioner James O’Neill said, adding that he was armed with a paint-ball gun and a pellet gun.
Of the eight people killed, six were pronounced dead at the scene and two more were pronounced dead at a nearby hospital, Mr O’Neill said.
US President Donald Trump called the attacker “a very sick and deranged person.”
He later tweeted: “We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!”
In the UK, Prime Minister Theresa May said she is “appalled by this cowardly attack” in New York City.
New York City Mayor Bill de Blasio described the attack “cowardly act of terror aimed at innocent civilians”.