At least one was killed and 10 others were injured when a van was driven into worshippers early morning on Monday near a north London mosque in what police have called a "major incident."
Police said the incident took place near Finsbury Park Mosque in Seven Sisters Road. A van driver who was arrested by police has been taken to hospital and is subject to a mental health assessment.
Describing it as a "terrible incident," British Prime Minister Theresa May said: "All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene."
The incident came weeks after three Islamist radicals rammed a van into pedestrians on London Bridge before rampaging through Borough Market with knives, killing eight people before being shot dead by police.
Labour leader Jeremy Corbyn, who lives nearby, said: "I'm totally shocked at the incident at Finsbury Park tonight. I've been in touch with the mosques, police and Islington Council regarding the incident."
"Officers are on scene with other emergency services. There are a number of casualties being worked on at the scene," the Met Police said in a statement.
"There has been one person arrested. Enquiries continue," it added.
The Sky TV reported that "at least 10 people" were hit by the van.
The Evening Standard newspaper said that the "rented" van swerved onto the pavement on Seven Sisters Road shortly after midnight. A man was then said to leap out of the vehicle, and unconfirmed reports say at least one of the victims was stabbed.
The newspaper said the arrested man is "white with black hair."
The Muslim Council of Britain (MCB) said a van "intentionally" ran over worshippers, the BBC reported.
Many of the victims are believed to have just left evening prayers after breaking the Ramadan fast.
London Ambulance Service said it had sent "a number of resources" to the scene.
One eyewitness who lives in a flat on Seven Sisters Road told the BBC she saw people "shouting and screaming," saying: "Everyone was shouting 'a van's hit people'."
London Ambulance Service deputy director of operations Kevin Bate said: "We have sent a number of ambulance crews, advance paramedics and specialist responses teams to the scene."
"We are working closely with other members of the emergency services at the scene," said Bate.
"Our priority is to assess the level and nature of injuries and ensure those in the most need are treated first and taken to hospital," said Bate.