Ten people were injured and a man also died at the scene after a van drove into a crowd of Muslim worshippers near a mosque in London in the early hours of Monday.
London Mayor Sadiq Khan called it an "horrific terrorist attack" while Muslim leaders linked the incident to a rise in Islamophobic crime.
Here is what we know:
- What happened? -
Witnesses said a white van struck a crowd of Muslim worshippers who had been attending evening prayers during the holy month of Ramadan and were looking after an elderly man who had collapsed in an unrelated incident.
Police said they were first called shortly after midnight.
Witness Khalid Amin told BBC television that the driver of the van was shouting: "I want to kill all Muslims."
- Where did it take place? -
The incident happened outside the Muslim Welfare House, 100 metres (yards) around the corner from the Finsbury Park Mosque. The mosque was once a notorious hub for radical Islamists that has since worked to become a centre for inter-faith outreach.
The scene is just metres from the main railway line linking London with Edinburgh, the main Arsenal Football Club shop and what was the Rainbow Theatre, considered the prestige London concert venue in the 1960s and 1970s.
- Who is the suspect? -
The white man driving the van was detained by members of the public and then arrested by police on suspicion of "the commission, preparation or instigation of terrorism including murder and attempted murder."
He was named in the British press as 47-year-old father of four, Darren Osborne. A property in the Welsh capital of Cardiff said by the media to be his home was searched by investigators.
London's Metropolitan Police said they would not name the suspect until he was charged.
- How many victims? -
The attack unfolded as an elderly man was receiving first aid from members of the public. He later died at the scene, though it is not yet clear whether his death was directly linked to the attack, said Neil Basu, the police senior national counter-terror coordinator.
Ten people were injured in the attack, all of them Muslims, with eight requiring hospital treatment. Two were in a very serious condition.
- Why are many residents angry? -
Many local Muslim worshippers complained that police did not immediately treat the attack as a terrorist incident, saying the response would have been different if it had been an Islamist assault.
Police issued an initial statement at 1:03 am (0003 GMT) saying a vehicle had collided with pedestrians.
In a statement at 4:46 am they said the incident was being investigated by counter-terrorism police.
Some people at the scene also complained that police took too long to arrive. One witness told the BBC he held down the suspect for 20 to 30 minutes before officers appeared.
But the police said they had responded "instantly" as officers were in the immediate vicinity, and that additional officers arrived within 10 minutes.
London police chief Cressida Dick later said the incident was "quite clearly an attack on Muslims" and promised a stepped-up police presence near mosques as the holy month of Ramadan draws to a close.
- How did Muslim groups react? -
The Muslim Council of Britain umbrella group said it expected increased security "as a matter of urgency".
"Over the past weeks and months, Muslims have endured many incidents of Islamophobia and this is the most violent manifestation to date."
Following an Islamist-inspired van-and-knife attack in the London Bridge area on June 3, the city saw a sharp rise in anti-Muslim crimes.
On June 6 alone, 20 anti-Muslim incidents were reported, compared with a daily average of 3.5 incidents previously in 2017.
Egypt's Al-Azhar institution, a leading authority of Sunni Islam, called it a "racist, sinful act" and urged "Western countries to take all precautionary measures to limit the phenomenon of Islamophobia."
- How did politicians react? -
British Prime Minister Theresa May vowed to fight extremism in all its forms and held an emergency cabinet meeting on Monday.
"There has been far too much tolerance of extremism... including Islamophobia," she said.
May condemned the assault as "sickening", saying Britain's determination to fight "terrorism, extremism and hatred... must be the same, whoever is responsible".
She visited the Finsbury Park Mosque where she met local faith leaders.
Mayor Khan called it a "deliberate" and "horrific terrorist attack" on "innocent Londoners".
France and Germany also condemned the attack and US President Donald Trump's daughter Ivanka expressed solidarity with the worshippers in a tweet but no comment so far from her father.