A police helicopter was seen circling overhead as photos posted on social media showed Seven Sisters Road in Finsbury Park cordoned off and numerous officers and police vehicles at the scene.
He said it was too early to say whether the person's death near the mosque was due to the attack by the van driver who ploughed into worshippers.
According to one man at the scene, the suspect - a 48-year-old who was detained by onlookers before being arrested by police - screamed "I'm going to kill all Muslims" before he was tackled to the floor.
British Home Secretary Amber Rudd says police "immediately" treated a fatal incident outside a London mosque as a suspected terrorist attack. The statement from her office said her thoughts were with the injured, their loved ones and emergency services who responded to the scene.
Witnesses said a van veered off the road into worshippers leaving Ramadan prayers in Finsbury Park just after midnight on Monday, the Independent newspaper reported.
Warfa said numerous people in the street were worshipers who'd been attending late-night taraweeh prayers at Finsbury Park Mosque.
A British Muslim group said the people struck down by the vehicle were worshipers leaving a mosque.
The Muslim Council of Britain said Monday's attack was the most violent manifestation of Islamophobia in Britain in recent months and called for extra security at places of worship.
The Sky News quoted the witnesses as saying that at least 10 people were hit by the van.
London Ambulance Service Deputy Director of Operations, Kevin Bate said: "We were called at 12.15am to reports of a road traffic collision at Seven Sisters Road".
BLOODSHED has come to the streets of Britain again after what is being treated as a terror attack in North London.
London vehicle hits pedestrians causing multiple casualties
Police say a vehicle struck pedestrians on the road in north London, leaving several casualties and one person has been arrested. They are very bad. "He was fighting to run away". "People were coming out of the mosque and they just drove through them".
Two persons were killed and 10 others injured on Monday when a van collided with pedestrians near a North London mosque in an incident which is being investigated by counter-terrorism officers, police said.
The man, who did not want to be named, said: "The gentleman went straight down this road, people were just conversing, talking, just doing what we're doing". Police said he was already receiving first aid when the attack unfolded.
Police said a suspect was quickly and calmly turned over to the police, and that no one else was found in the van.
Mahroof Mohammed said he saw several injured people. Thank God I just moved to the side, I just jumped.
She added: "I saw a lot of people injured".
The Finsbury Park Mosque opened in 1994 and became a hotbed of Islamist militants, including Zacarias Moussaoui, a Frenchman convicted of conspiring to kill Americans as part of the September 11, 2001, attacks, and Richard Reid, who attempted to down a USA jetliner in late 2001 with explosives packed in his shoes.
A witness, Abdiqadir Warra, told AFP that the van "drove at people" and that some of the victims were carried for several metres along the road.
The attack comes at a time of political turmoil in Britain, as May, weakened by the loss of her parliamentary majority in a June 8 election, plunges into divorce talks with the European Union.
Opposition Labour leader Jeremy Corbyn, the local MP for the area, said he would attend prayers later in the day at the mosque.
In March, a terrorist drove into pedestrians on Westminster Bridge in London, killing four, and fatally stabbed a policeman guarding the gates of parliament before being shot dead by armed officers.
Manchester was also hit by a severe attack when a bomber killed more than 20 people at an Ariana Grande concert.