One person has died at the scene and several people have been injured after a van rammed into worshippers leaving a mosque in North London.
The Muslim Council of Britain says the vehicle hit people as they were leaving the Finsbury Park Mosque, one of Britain's largest. The attack comes during the Muslim holy month of Ramadan, when people attend prayers at night.
London police say one man has been pronounced dead at the scene and eight have been taken to hospital.
The Muslim Council of Britain has condemned the attack, calling it "the most violent manifestation" yet of Islamophobia and calling for extra security around mosques.
Police were called just after 12.20 am to reports of 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.
"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'," one 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."
The incident follows a series of attacks in Britain in recent months blamed on Islamist militants, the latest just over two weeks ago in which three men drove into pedestrians on London Bridge and stabbed people at nearby restaurants and bars, killing eight.
One witness told CNN it was clear that the attacker at Finsbury Park had deliberately targeted Muslims.
"He tried to kill a lot of people so obviously it's a terrorist attack. He targeted Muslims this time," the witness, identified only as Rayan, said.
Other witnesses told Sky television that the van had hit at least 10 people.
Miqdaad Versi, assistant secretary general of the Muslim Council of Britain, said the van had deliberately swerved into a group of people who were helping a man who was ill and had fallen to the ground.
"A number of passers-by, or friends, or people who had come by from the mosque, were gathering around him to help take him to his family, take him to his house," Versi told Reuters.
"At that moment in time, basically a van swerved into them deliberately," he said, citing a witness at the scene.
He said the driver had run out of the van but a group of people caught him and held him until police arrived.
A Reuters witness saw at least one person being loaded into an ambulance. Armed police, ambulances and the fire service were in attendance.
British Prime Minister Theresa May says her thoughts are with all those injured.
"This is a terrible incident," she said in an emailed statement. "All my thoughts are with those who have been injured, their loved ones, and the emergency services on the scene."
Earlier, Labour leader Jeremy Corbyn expressed his shock at what had happened, writing on Twitter: "I'm totally shocked at the incident at Finsbury Park tonight.