UK Prime Minister Theresa May has described the terror attack near a London mosque as sickening as others in recent weeks.
A man drove a van into worshippers close to Muslim Welfare House in Finsbury Park as they were gathered to help an elderly man who had collapsed.
He later died, but it is not clear if this was a result of the attack.
Nine other people were taken to hospital.
A 47 year-old man was held on suspicion of attempted murder and later further arrested over alleged terror offences.
Scotland Yard said he was being held on suspicion of the commission, preparation or instigation of terrorism including murder and attempted murder.
Searches are also being carried out at an address in the Cardiff area.
Met Police Commissioner Cressida Dick said the incident in north London 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.