A man has died and 10 people have been injured after a man drove a van into worshippers near a north London mosque.
Eight people were taken to hospital after the “terrorist attack” near Finsbury Park Mosque, police said.
A group of people were helping a man who had collapsed when they were hit by the van just after midnight. The man has died, but it is unclear whether this was as a result of the attack.
A 48-year-old man has been arrested on suspicion of attempted murder.
Police said all the victims of the attack, which took place outside Muslim Welfare House, were Muslim.
Eyewitness Abdul Rahman said the driver had said he wanted to “kill all Muslims” and told the BBC he struck the man and helped subdue him.
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Metropolitan Police’s deputy assistant commissioner Neil Basu said the “terrorist attack” began when the van was driven into a man who was already being given first aid by the public on the pavement.
“Sadly that man has died. It is too early to state if his death was a result of this attack.”
Several of those in hospital are “seriously injured” and there are currently no other suspects, he added.
Earlier, police also said the arrested man would be “subject of a mental health assessment in due course”.
Extra police are being deployed to reassure communities, especially those observing Ramadan, Mayor of London Sadiq Khan said.
He also said his “thoughts and prayers” were with those affected by the “horrific terrorist attack on innocent people”.
“While this appears to be an attack on a particular community, like the terrible attacks in Manchester, Westminster and London Bridge it is also an assault on all our shared values of tolerance, freedom and respect.”
Forensics officers are examining a white van which has Pontyclun Van Hire on it – a firm from Rhondda Cynon Taff in south Wales.
At the scene
By Cherry Wilson, BBC News
Locals say this is a proudly multicultural area, where the biggest rivalry is whether you support Arsenal or their north London rivals, Tottenham.
Now the mood here is one of shock, as residents stand by the police cordon seeing the aftermath of yet another attack in London.
Mother-of-four Nicola Senior, 43, is walking back from taking her children to school when she stops to take in the scene.
She said: “I’m frightened. Is there going to be retaliation?
“I am fearful for my kids. Can we go to the park? Can we go to the church? It feels like this is happening all the time.”
Mr Rahman, an eyewitness, said: “When the guy came out from his van he wanted to escape, run away and he was saying ‘I want to kill Muslims. ‘I want to kill Muslims.’
“I hit him on his stomach… and then me and the other guys… we held him to the ground until he couldn’t move. We stopped him until the police came.”
Another witness, who gave his name as Abdul, told the BBC the arrested man was shouting “kill me, I’ve done my job”.
The Muslim Council of Britain said this was the “most violent manifestation to date” of recent Islamophobic incidents.
“We expect the authorities to increase security outside mosques as a matter of urgency.”
Many of the victims are believed to have just left evening prayers at the Muslim Welfare House after breaking the Ramadan fast.
It has appealed for calm, adding that “all of our efforts should be towards getting justice for the victims and ensuring our community stays the diverse, tolerant and welcome place we know it to be”.
Prime Minister Theresa May is chairing a meeting of the emergency Cobra committee, after which she will give a statement.
She described it as a “terrible incident”, adding: “All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene.”
This is the fourth terror attack in the UK in four months, after incidents in Westminster, Manchester and on London Bridge.
The van driver was arrested after being detained by members of the public at the scene following the collision at 00:20 BST.
An eyewitness told how he jumped out the way of the van as it struck.
He said: “He just came into all of us. There was a lot of people. We got told to move straight away.
“I was shocked, shocked, shocked. There were bodies around me.”
BBC London’s Sarah Lee, at the scene, said the atmosphere around Finsbury Park was “one of anger”.
“People of all nationalities are expressing their concerns and frustration at the latest attack on the city.
“It’s clear people are concerned about possible revenge attacks in the upcoming weeks.”
Home Secretary Amber Rudd described it as an appalling incident, and said new funding for security at religious sites had recently been arranged.
“We have a places of worship fund which we announced last summer, which is there to protect places of worship like mosques. We will make sure that we do all we can to reduce these sort of attacks.”
By Dominic Casciani, BBC home affairs correspondent
This is not the first time that Muslims have been targeted in an apparent act of terrorism in the UK – and all the signs are that this terrible incident is nothing short of that.
The threat from extreme right-wing groups has been growing in recent years – 16% of all terror arrests in the year to March were classed as “domestic extremism”.
Those who have turned to violence have tended to go for visible Muslim targets – namely mosques.
In 2013 an extreme right-wing Ukrainian man murdered a Muslim grandfather in Birmingham and tried to bomb three mosques.
The following year saw the jailing for 10 years of a man from north-west England who was researching bomb-making and mosques to target.
And just last December the home secretary banned “National Action”, a group whose supporters have been investigated for planning violence.
Environment Secretary Michael Gove said the attack was a “disgusting and immoral act”.
“Words fail me that someone could find it in their heart to behave in this way.”
Labour leader and Islington North MP Jeremy Corbyn has visited the area.
He told the BBC that “an attack on a mosque, an attack on a synagogue, an attack on a church is actually an attack on all of us”.
“We have to protect each other’s faith, each other’s way of life, and that’s what makes us a strong society and community.
“The only answer to this has to be strong communities together, so these dreadful incidents don’t happen.”
Mr Corbyn will be attending prayers at Finsbury Park Mosque later on Monday with Islington Council Leader Richard Watts.
Communities Secretary Sajid Javid is also visiting the area.
Labour’s shadow home secretary and Hackney North and Stoke Newington MP Diane Abbott said on Twitter that police “must urgently review security for all mosques”.
Outgoing Liberal Democrat leader Tim Farron said the attack was “senseless barbarism”.
Green Party co-leader Caroline Lucas said she was thinking of the community and emergency services, adding: “We will not be divided.”
Witness Cynthia Vanzella said on Twitter: “Horrible to watch police officers doing cardiac massage at people on the floor, desperately trying to save them. I just hope they did.”
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Finsbury Park attack: Man dies as van hits worshippers