The driver of the van - a 48-year old - has been arrested on suspicion of murder, he added.
Today's incident followed a series of attacks in Britain.
The vehicle was rented from a Welsh company known as Pontyclun Van Hire.
Witness Abdiqadir Warra told AFP the van "drove at people" and some of the victims were carried for several metres along the road.
Police said another man died at the scene, although he was receiving first aid at the time and it wasn't clear if he died as a result of the attack or of something else.
The Finsbury Park mosque was associated with extremist ideology for several years after the September 11, 2001, attacks in the United States, but was shut down and reorganized.
Another eyewitness, who gave his name as Athman, told AFP: "People were screaming: 'It's a terrorist attack, it's an attack, this guy attacked us, ' and I had to run back and tried to save people who were still alive and giving water, helping the police".
A witness who lives opposite the scene of the incident told the BBC a white van stopped near the Finsbury Park Mosque, as Muslims celebrated Ramadan in the multiethnic area. "I didn't see the attacker himself, although he seems to have been arrested, but I did see the van".
London Mayor Sadiq Khan said it was a "horrific terrorist attack", calling it "deliberate" and aimed at "innocent Londoners, many of whom were finishing prayers during the holy month of Ramadan".
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While a 48-year-old man was arrested at the scene of the attack, according to the Met Police, some witnesses reported seeing three attackers, including two who fled the scene.
Prime Minister Theresa May, who was criticised for her reaction to London's Grenfell Tower fire disaster last week, responded quickly, saying police were treating it as "a potential terrorist attack".
In a statement, police said eight people were hospitalized while two others were treated at the scene.
The incident comes at a time when emotions are high in England, where there have been several recent terrorist attacks in recent weeks.
Following an Islamist-inspired van-and-knife attack in the London Bridge area on June 3, the city saw a sharp rise in anti-Muslim crimes.
The interior minister, Amber Rudd, said police "immediately" treated the incident as a suspected terrorist attack.
The Muslim Council of Britain called what happened a "violent manifestation of Islamophobia".
The Muslim Council of Britain, an advocacy group for the country's Muslim community, said in a Monday tweet that the incident took place near the Muslim Welfare House - on the opposite side of the road - not outside the Finsbury Park Mosque itself.
Finsbury Park mosque was once infamous as the stamping ground of hook-handed hate preacher Abu Hamza.
The local MP is Jeremy Corbyn, the leader of the UK's main opposition Labour Party.
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