Prime Minister Theresa May says the terror attack near a north London mosque is "every bit as sickening" as other recent ones to hit the UK.
A man drove a van into worshippers close to the Muslim Welfare House in Finsbury Park as they were gathered to help an elderly man who had collapsed.
That man later died, but it is not clear if his death was the result of the attack. Ten others were also hurt.
A 48-year-old man has been arrested on suspicion of attempted murder.
Metropolitan Police Commissioner Cressida Dick said the incident was "quite clearly an attack on Muslims", and the community would now see more police, including armed officers, in the area, "particularly around religious establishments".
It is the fourth terror attack in the UK in three months, after incidents in Westminster, Manchester and on London Bridge.
Police said all the victims of the attack were Muslim and many were believed to have just left evening prayers after breaking the Ramadan fast.