One person is dead and an emergency response has been launched, after what could be the third terror attack on British soil in only a month.

Witnesses say three people were inside a van, when it was driven into a crowd of worshippers leaving Ramadan evening prayers at London's Finsbury Park Mosque.

A 48-year-old accused of driving the van has been arrested, is being treated in hospital, and will undergo a mental health assessment.

Met Police say eight people have been taken to three separate hospitals, and paramedics treated two people at the scene for minor injuries.

British prime minister Theresa May says the incident is being treated as a potential terrorist attack, and she will chair a meeting to plan the emergency response.

Britain's Muslim Council said the van driver targeted Muslim worshippers during the holy month of Ramadan.

Islamic Monthly senior editor Asalan Iftikhar told CNN if the incident was deliberate, it was an horrific act of terrorism against Britain's Muslim population.

"Every night during the Islamic holy month of Ramadan we have our nightly prayers, where hundreds of people come to their locals Mosque's to p[ray with their fellow Muslims."

Police were alerted just after 12.20am on Monday local time to a collision on Seven Sisters Road, which runs through the Finsbury Park area of the city.

They said there were a number of casualties and one person had been arrested.

London Ambulance Service issues statement regarding incident in London https://t.co/sikMGBjVYr pic.twitter.com/cZxFd6FheP

— CNN (@CNN) June 19, 2017

Cynthia Vanzella witnessed the carnage from her window.

She told CNN people were screaming for help, and many people were seriously injured.

"At least two of them on the floor, one of them I think maybe was really really badly hurt because I thought police officer's were trying to resuscitate them."

A witness told Sky News at least ten people were hit by the van.

"From the window, I started hearing a lot of yelling and screeching, a lot of chaos outside. ... Everybody was shouting: 'A van's hit people, a van's hit people'," a woman who lives opposite the scene told the BBC.

"There was this white van stopped outside Finsbury Park mosque that seemed to have hit people who were coming out after prayers had finished. I didn't see the attacker himself, although he seems to have been arrested, but I did see the van."

Mohammed Kozbar, chairman of the mosque, told The Sun: "Whoever did this, he did it to hurt people and it's a terrorist attack.

"We call it a terrorist attack as we called it in Manchester, Westminster and London Bridge."

A Reuters witness saw at least one person being loaded into an ambulance. A number of police and ambulances were in attendance.

Other witnesses described the scene on Twitter.

"Horrible to watch police officers doing cardiac massage at people on the floor, desperately trying to save them. I just hope they did," Cynthia Vanzella said.

Finsbury Park mosque used to be infamous as the stamping ground of hook-handed hate preacher Abu Hamza.

A number of terrorists were linked to the mosque, including shoebomber Richard Reid, who attempted to detonate explosives on an American Airlines flight from Paris to Miami in 2001.

It was raided and shut down and later reclaimed by the local Muslim community, who have transformed it into a place which actively promotes better community relations across faiths.

The 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.

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.

On May 22, a suicide bomber killed 22 people at a concert by American pop singer Ariana Grande in Manchester in northern England.

This racist bastard ran over innocent civilians on their way home from taraweh #FinsburyPark pic.twitter.com/YHvsVZHqIY

— Didier (@Known_As_H) June 18, 2017

Anyone an idea what exactly happened? @metpoliceuk #finsburypark #sevensisters pic.twitter.com/d4VoQtcJZr

— Thomas Van Hulle (@Thomasvanhulle) June 18, 2017

#FinsburyPark lots of police on scene outside the Finsbury Park Mosque including armed units pic.twitter.com/si5CIPUIb4

— London 999 Feed (@999London) June 18, 2017