At least one person has died and ten were injured when a van plowed into a crowd gathered outside a London mosque after prayers.
London mayor Sadiq Khan said police were responding to a "horrific terrorist attack."
Prime Minister Theresa May called it a "potential terrorist attack" and said: "All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene.”
The Metropolitan Police called it a "major incident" and said an investigation was being carried out by the Counter Terrorism Command. Eyewitnesses said there were a total of three "attackers," one of whom was apprehended while two more fled the scene. Police said they arrested a 48-year-old man at the scene and that no other suspects had been identified.
Two people were treated at the scene and eight were transferred to three London hospitals, according to London Ambulance Service Deputy Director of Operations, Kevin Bate.
The incident happened in Finsbury Park in the north of London, in Seven Sisters Road, according to officials.
May will chair an emergency meeting on Monday morning to coordinate the government's response.
The Muslim Council of Britain called the incident a "terror attack" and the "most violent manifestation" of Islamophobia.
Eyewitnesses interviewed by ABC News said people were gathered outside the mosque after prayers tending to an old man who was having a heart attack when the van drove into them. There were men, old men and women, no children.