The man who collapsed was pronounced dead at the scene and 10 people were injured, with eight taken to hospital.
A man identified as Darren Osborne, a 47-year-old Welshman from Cardiff, was allegedly behind the wheel.
His next-door neighbors, who are Muslim, said that Osborne had approached their 12-year-old son in the road and called him "inbred" this past weekend.
Officers close by attended the scene instantly, the attack was declared as a terrorist incident within eight minutes and armed officers were on the scene within 10 minutes, police said.
"You really had a mosque there that was essentially a way station for worldwide jihadism", said Raffaello Pantucci, the director for global security studies at the Royal United Services Institute in London.
At the time of his arrest, while waiting in a police van, Osborne was apparently "blowing kisses" to the angry crowd surrounding him. Police are carrying out searches at an address in the Cardiff area.
- British Chief Rabbi Ephraim Mirvis, who hosted an Iftar at his home last night for the local Muslim community, said "we must never allow hatred to breed hatred".
She later visited Finsbury Park Mosque where she held talks with faith leaders.
On June 3, three men had driven a van into pedestrians before embarking on a stabbing spree.
"This is an appalling attack on a wonderful, happy, integrated community here in London".
Metropolitan Police Deputy Assistant Commissioner Neil Basu said police are investigating whether the death of the man getting first aid was a direct result of the attack, but it was too early to say for sure.
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The specialist bereavement support for families will be provided by the NHS, Jeremy Hunt, Secretary of State for Health, said. She said after the meeting that there have been "huge frustrations" in the community as people tried to get information.
The mosque now claims to attract nearly 2,000 worshippers and in 2014 it won a prestigious award for its services to the community, though its past links to extremism made it a target for criticism from Britain's far right.
When several angry witnesses attempted to beat up the 47-year-old attacker who plowed his auto into a group of worshipers outside the Finsbury Park Mosque, an imam stepped in to prevent the man from being harmed, shouting, "No one touch him - no one! We will stay a strong city", the mayor said.
London police say one suspect has been detained in an apparent early-morning attack on worshippers standing outside a mosque.
Mohammed Mahmoud told reporters: "We told them the situation - there's a man, he's restrained, he mowed down a group of people with his van and there is a mob attempting to hurt him and if you don't take him then, God forbid, he might be seriously hurt".
Eyewitnesses reported chaotic scenes as the incident unfolded just after midnight, when worshipers had finished evening Ramadan prayers.
- Muslim Council of Britain said in a statment that "ordinary British citizens were set upon while they were going about their lives, completing their night worship".
Video posted online of the aftermath of Monday's attack showed a scene of chaos as people tried to help the injured. "We treat this as a terrorist attack and we in the Met are as shocked as anybody in this local community or across the country at what has happened".
Mr Khan said the attack, like those perpetrated in Manchester and London Bridge, appeared to be targeting a particular community and was "an assault on all our shared values of tolerance, freedom and respect".
The attack occurred outside the Muslim Welfare House, a small mosque with about 200 congregants.
Britain's terror alert level is at "severe", meaning security officials believe an attack is highly likely. The attack Monday hits a community already feeling targeted in the fallout from the London Bridge killings and other attacks blamed on Islamic extremists.