"This was an attack on London and all Londoners and we should all stand together against extremists whatever their cause", saidNeil Basu, senior counter-terrorism officer for the Metropolitan Police.
The man who died was was already receiving first aid at the time of the attack, the latest in a spate of terrorist strikes in Britain.
The worshipers were attending to an elderly man who'd fallen suddenly ill after evening prayers when terror struck.
The East Londonmosque said its buildings were evacuated and police were called after a telephone threat, which was later confirmed as a hoax bomb threat. Police say they're treating it as a terrorist incident.
A witness, Abdiqadir Warra, said the driver of the van "drove at people" and that some of the victims were carried for several metres along the road.
Authorities said the incident was being treated as a terror attack. He later died but it was not clear if it was because of the attack.
Witness Hussain Ali added the suspect was smiling and waving as he brought carnage to Seven Sisters Road in Finsbury Park, north London. He was taken to a hospital as a precaution, where he'll receive a mental health assessment, according to the statement.
Another eyewitness, who gave his name as Athman, toldAFP: "While he was in the police van he was waving for victory, he was very happy".
The use of a vehicle to mow down pedestrians drew horrifying parallels with this month's London Bridge attack, when three men drove a van into pedestrians before embarking on a stabbing spree, and with another auto and knife rampage near parliament in March. He was arrested on suspicion of the commission, preparation or instigation of terrorism including murder and attempted murder.
They said more officers would be deployed to provide reassurance to mosques.
Then, something remarkable happened: "An imam from the mosque outside of which the attack took place came outside and persuaded the angry, grief-stricken crowd to practice peace, not violence".
His mother, Christine Osborne, 72, told the Guardian: "I'm sorry, I'm not talking to reporters". The mosque, which today operates largely as a community center, rose to global notoriety in the early 2000s, due to its links with Egyptian-born radical cleric Abu Hamza al-Masri.
"While this appears to be an attack on a particular community, like the bad attacks in Manchester, Westminster and London Bridge, it is also an assault on all our shared values of tolerance, freedom and respect", Khan said.