The attack took place just after midnight outside the Muslim Welfare House, which houses a mosque, on the Seven Sisters Road in the northern part of the city, the Guardian reported. The site of the incident is close to another mosque near the Finsbury Park on the same road.
Several worshippers were in the area at the time of incident after attending Ramzan prayers.
Counter-terror police were investigating the incident.
The Metropolitan Police said that one man was pronounced dead at the scene and eight others were injured.
Prime Minister May said police were treating the van incident “as a potential terrorist attack”.
Earlier she described it as a “terrible incident”, adding: “All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene.”
“The driver of the van – a man aged 48 – was found detained by members of public at the scene and then arrested by police in connection with the incident,” a spokesperson for the Metropolitan Police said.
“He has been taken to hospital as a precaution, and will be taken into custody once discharged. He will also be subject of a mental health assessment in due course.”
Scotland Yard said armed police were at the scene, with the investigation being carried out by Counter Terrorism Command and an extra officers deployed to reassure Muslims during Ramzan.
The Metropolitan Police earlier declared the collision as a “major incident” and said that there were “a number of casualties”.
Harun Khan, the head of the Muslim Council of Britain, tweeted that the van had “intentionally” run over people leaving night prayers.
According to the Independent, footage of the incident showed injured people motionless on the pavement as angry crowds surrounded a white man believed to be the driver.
The Guardian quoted an eyewitness, Boubou Sougou, as saying: “It was not an accident, I saw everything. People were badly injured. The van driver tried to escape but people grabbed him. He did not say anything.”
London’s transport authority said on Twitter that the Seven Sisters road had been closed due to an “emergency services incident”.
The city’s ambulance service said in a statement on Twitter that it had sent “a number of resources” including ambulance crews, paramedics and specialist response teams to the scene.
“An advance trauma team from London’s Air Ambulance has also been dispatched by car,” the statement said.
Another eyewitness, who lives in a flat on the Seven Sisters Road, said the van seemed to have hit people who were coming out of the Finsbury Park mosque after prayers finished, the BBC reported.
Witnesses said the road was “backed up” with police cars, ambulance and fire engines.
This incident followed a series of attacks in Britain.
Eight people were killed and 50 injured on June 3 when three Islamist militants drove into pedestrians on London Bridge and stabbed people at nearby restaurants and bars.
Two weeks earlier, a suicide bomber killed 22 people at a concert by American pop singer Ariana Grande in Manchester in northern England.
On March 22, a man drove a rented car into pedestrians on Westminster Bridge in London and stabbed a policeman to death before being shot dead. His attack killed five people.