The attacker who drove a van into worshippers outside Finsbury Park Mosque has been identified as a 47-year-old from Wales who authorities say had no known links to extremist groups.
British media have identified the attacker as Darren Osborne of Cardiff, Wales.
He is being held for terrorism offences and attempted murder, London's Metropolitan Police said.
One man has been confirmed dead and 10 were others injured in the attack, which occurred just after midnight on Sunday local time as worshippers were leaving following Ramadan prayers.
A witness said the driver of the van that ploughed into people shouted "kill all Muslims".
"He was saying, 'I wanna kill more people, I wanna kill more Muslims'," Abdulrahman Aidroos told the BBC.
Mr Aidroos said he had helped tackle and detain the driver while other witnesses said an imam stepped in to ensure the man was not harmed.
"We found that a group of people quickly started to collect around him, around the assailant and some tried to hit him either with kicks or punches," imam Mohammed Mahmoud said.
"By God's grace we managed to surround him and to protect him from any harm."
One official said the Muslim cleric's "bravery and courage" possibly saved the man's life.
Prime Minister Theresa May visited the Finsbury mosque the day after the attack.
Ms May said the attack was "every bit as sickening as those which have come before".
"It is a reminder that terrorism, extremism and hatred take many forms and our determination to tackle them must be the same, whoever is responsible," she said.
London Mayor Sadiq Khan said there would be an increased police presence around mosques in coming days to protect Muslims from hate crimes.
He said there had been a spike in Islamophobic hate crimes since the London Bridge terrorist attack.
"Don't think it's too trivial. Don't think the police won't take action, they will."
British Security Minister Ben Wallace said the man who drove the van into worshippers was not previously known to authorities for any links to extremism.
"This man was not known to the authorities in the space of extremism or far-right extremism," Mr Wallace told Sky News.
London's Police Commissioner Cressida Dick said the attack was clearly aimed at Muslims.
"Extra officers are on duty in the area to help reassure the local community. They will be there for as long as they are needed," she said.
"Communities will see additional officers patrolling across the city and at Muslim places of worship.
"We are working as hard as we can to protect all our communities, and we will not be defeated."
Deputy Assistant Commissioner Neil Basu said it was not yet clear whether the man who died at the scene was killed because of the attack.
"The attack unfolded as a man was already receiving first aid at the scene, sadly that man has died," Mr Basu said earlier.