Video report by ITV News Correspondent Neil Connery
A 47-year-old man has been arrested after a van ploughed into worshippers outside a mosque which police have said was "quite clearly an attack on Muslims".
Darren Osborne, described as a large white man, was held at the scene on suspicion of murder and attempted murder.
Scotland Yard said he had been further arrested on suspicion of the commission, preparation or instigation of terrorism.
They added searches were being carried out at an address in Cardiff in connection with the attack.
One man died and 10 were injured when a van ploughed into pedestrians near Finsbury Park Mosque in north London in the early hours of Monday.
Witnesses said they heard the driver shout: "I'm going to kill Muslims" before he was detained by members of the public.
Police are still working to determine if the man who died was killed in the attack, as he had collapsed beforehand and was receiving medical treatment outside the Muslim Welfare House.
Abdulrahman Aidroos, who tackled the suspect, told ITV's This Morning the man asked him to kill him while being pinned down on the ground before police arrived.
The suspect was later seen blowing kisses to the crowd while being put into a police van.
The Muslim Welfare House hailed its imam, Mohammed Mahmoud, who helped to calm the situation as the attacker was first grabbed.
The imam guarded the man for around 20 minutes until police arrived, telling angered onlookers: "Do not touch him."
Prime Minister Theresa May praised the bravery of those who detained the suspect as she condemned the attack as "sickening" in a statement delivered at Downing Street.
After holding an emergency Cobra meeting over the attack in Seven Sisters Road, she said the early assessment by police suggested the attacker "acted alone".
Mrs May, who later visited the Finsbury Park Mosque, said her recent comments that the nation showed too much tolerance of extremism in the wake of the London and Manchester terror attacks included Islamophobia.
"The terrible terrorist attack that happened last night was an evil act born out of hatred and it has devastated a community.
"I'm pleased to be here today to see the strength of that community coming together all faiths, united in one desire, to see extremism and hatred of all sorts driven out of our society.
"There is no place for this hatred in our country today."
Mrs May added counter-terrorism strategy will be reviewed to see if police and security services have the powers they need amid plans for a new Commission for Countering Extremism.
Labour leader and local MP Jeremy Corbyn, who also visited the mosque, condemned the "terror on the streets" as he visited the site of the attack within his constituency.
"An attack on a mosque, an attack on a synagogue, an attack on a church is actually an attack on all of us," he said.
"When somebody drove a truck into a crowd of people who were looking after somebody who had fallen ill on the road.
"People injured, some of them very seriously, as a result of that.
"The man concerned has been arrested, is under questioning and will no doubt be charged and eventually be brought to trial."
Eight of the injured were taken to three hospitals, while two people were treated at the cordoned-off scene for minor injuries.
All were from the Muslim community.
Many of those injured were believed to have been leaving the Finsbury Park Mosque after prayers following the end of their daily Ramadan fast.
London Mayor Sadiq Khan said the attack came amid an "unprecedented" and "terrible" few weeks for the capital following the Grenfell Tower fire and other recent terror attacks - but insisted London would remain "united".
The Welsh van hire company whose vehicle was used in the attack said it was "cooperating fully" with the police investigation.
"We at Pontyclun Van Hire are shocked and saddened by the incident that took place at Finsbury Park last night," the firm said in a statement.
"All our thoughts are with those who have been injured in this cowardly attack."
ITV News Wales Correspondent Rupert Evelyn, who filmed police at the hire site in Mid Glamorgan, said it was still not known if the white van was hired or stolen.
In a statement posted on its website, the Finsbury Park Mosque said it "condemns in the strongest terms a heinous terrorist attack".
"The van driver deliberately mowed down Muslim men and women leaving late evening prayers from Finsbury Park Mosque and Muslim Welfare House just after midnight," the statement said.
"This is a callous terrorist attack, which coincides with the murdered MP, Jo Cox, anniversary."
The Muslim Council of Britain has called for extra security around mosques after an attack that demonstrated "the most violent manifestation" of Islamophobia.
Home Secretary Amber Rudd said part of £2.5m security funds pledged by the government last summer were still available for places of worship to apply for additional protection.
She also played down concerns it took police several hours to publicly recognise it as an attack after it was initially reported as a road traffic incident.
The chairman of the Sikh Federation UK, Bhai Amrik Singh, said the "incidents in the last three months" should prompt "honest dialogue and a fundamental shift in the way government tackles all forms of hate and terror".
The attack follows a van-and-knife attack in London Bridge on June 3 that killed eight, the Manchester suicide attack on May 22 that killed 22 people and March 22's Westminster car-and-knife attack in which five died.
It was also just over a year to the day that Labour MP Jo Cox was shot and fatally stabbed by right-wing extremist Thomas Mair outside her constituency office in Batley, West Yorkshire on June 16, 2016.
Anyone with information about the Finsbury Park attack is urged to call the counter-terrorism number on 0800 789 321