In the second terror attack within a month in the US, a man wielding a truck as a weapon on Tuesday ploughed through bicyclists and pedestrians on a street near the site of the 9/11 attack killing eight persons - most of them tourists.

New York Mayor Bill De Blasio declared: "This was an act of terror, particularly cowardly act of terror aimed at innocent civilians."

An Uzbek immigrant, Sayfullo Saipov, 29, who drove the rented truck, was shot and captured alive by police and is in hospital after the attack that also left 11 others injured.

Police Commissioner James O'Neil said that one of the indications of a terror link was "a statement when he exited the vehicle". Witnesses said Saipov shouted "Allahu Akbar".

Police reported finding notes left by the alleged attacker vowing allegiance to the Islamic State (IS).

This was the first major foreign-inspired terrorist attack in the US after President Donald Trump took over with a vow to stamp out terrorism.

Five tourists killed were from Argentina and another person who died was from Belgium.

After the van hit a school bus, Saipov came out of it waving what appeared to be two weapons, which turned out to be a paintball gun and a pellet gun, police said. 

Two devices inside the van that looked like bombs were found to be fake, authorities said.

As of Tuesday night officials were not able say if Saipov was part of a network or was a "lone wolf" acting alone.

October began with the killing of 58 people at a concert in Las Vegas by a lone gunman in a hotel room. Authorities have been unable to find a motive for that attack by Stephen Paddock.

Tuesday's attack was a text book case of the strategy of using vehicles that the IS has been preaching as a way for people without access to guns or bombs to wage "jihad".

In August, 14 persons were killed by terrorists using a van in Barcelona, Spain, and London saw two attacks with cars this year. 

The deadliest such attack was in Nice, France, on July 14, 2016 in which 86 people were killed.

Saipov's mile-long rampage left a trail of smashed bicycles and blood in a mostly residential area with several educational institutions, about 400 metres from the World Trade Centre that was attacked on September 11, 2001, by terrorists on a hijacked plane killing about 2,750 people.

Tuesday's terror strike occurred just hours before the annual Halloween parade was to take place nearby.

"New Yorkers are resilient, our spirit will never be moved," de Blasio declared at a news conference.

And in a display of defiance against terror, New Yorkers later marched in the scheduled Halloween parade with de Blasio and Governor Andrew Cuomo leading the procession of costumed princesses, ghosts, zombies, superheroes and other characters from myth and entertainment.

Trump tweeted: "We must not allow IS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"

Experts in terrorism have said that after the IS recently lost its bases in Raqqa, Syria and in Iraq, it would increase attacks in the West.

While the New York attack can be viewed as a setback for Trump, he can claim that his anti-terrorism efforts have met with opposition.

He can use the attack as an argument for his plans for restricting immigration and strict scrutiny of immigrants. He wants to abolish the programme that holds a lottery to give green cards to people from countries that send few immigrants, which Saipov used to immigrate.

While his repeated attempts to restrict immigration from some countries that he says pose a risk have been blocked by courts, Uzbekistan was not on his list.

Indian Prime Minister Narendra Modi joined world leaders in condemning the terrorist strike, tweeting, "Strongly condemn the terror attack in New York City. My thoughts are with the families of the deceased and prayers with those injured."

A South Asian student made a video of the last moments of Saipov's rampage and his capture was broadcast by several TV stations. He said that at first he thought it was a Halloween prank but when he realised the bullets were real, he ducked.

The attack happened around the time the nearby schools were ending the day. Of those injured, two were students and another was a school employee who were on the bus that was hit by the van.

The schools were put in a lockdown with students herded back in and kept inside for more than an hour while parents waited outside anxiously.

Saipov, who had come to the US in 2010, had lived in Florida and of late moved to Patterson in New Jersey.

Two brothers from another former Soviet republic, Kyrgyzstan, had set off bombs made with pressure cookers at the Boston Marathon in 2013. Three people were killed and about 500 injured in the attack.

In May, a car was driven into pedestrians in Times Square killing a tourist and injuring about 20 people. Authorities declared that the driver, ex-Navy man Richard Rojas, was not a terrorist.

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