A vehicle drove into pedestrians near a mosque in north London, killing at least one person, early on Monday. One suspect has been taken into custody, as London's mayor says extra police have been called in.
Eyewitnesses reported seeing a group of people giving an emergency heart massage to one man before the attack happened.
Eight others were taken to hospital and two were treated at the scene, Basu said, adding that all of the victims were Muslim.
The driver of the van - a 48-year old - has been arrested on suspicion of murder, he added.
Video filmed immediately after the attack showed a Caucasian man being detained by police.
"The driver of the van...was found detained by members of the public at the scene and then arrested by police in connection with the incident," an earlier police statement said. Ambulance crews said ten people had been transferred to hospital.
The interior minister, Amber Rudd, said police "immediately" treated the incident as a suspected terrorist attack. Rudd, who is in charge for government law enforcement, said: "Londoners have been hit with a series of attacks and have been nothing short of heroic."
"We must all continue to stand together, resolute, against all those who try to divide us and spread hate and fear," Rudd said.
Prime Minister Theresa May - who said earlier on Monday that the incident could have been a terrorist attack - will chair an emergency meeting later this morning.
The chairman of the Finsbury Park Mosque, Mohammed Kozbar, complained that the "mainstream media" had been unwilling to call the attack a terrorist incident for many hours.
The leader of Britain's opposition Labour Party, Jeremy Corbyn - in whose constituency the attack took place - said he was "totally shocked" by the attack. He said in a Tweet he has been in touch with mosques, the police and the local council regarding the incident.
As it happened
Police said they were called just after 12:20 a.m. (2320 GMT Sunday) to reports of a collision on Seven Sisters Road, which runs through the Finsbury Park area of north London.
A witness who lives opposite the scene of the incident told the BBC a white van stopped near the Finsbury Park Mosque, as Muslims celebrated Ramadan in the multiethnic area.
"From the window, I started hearing a lot of yelling and screeching, a lot of chaos outside…Everybody was shouting: 'A van's hit people, a van's hit people'," she said. "I didn't see the attacker himself, although he seems to have been arrested, but I did see the van."
One witness, Abdul Rahman, told the BBC he saw the van "deliberately run over about 10 or 15 people." Rahman said he and another man wrestled the suspect to the ground and held him down for 20 to 30 minutes before police arrived.
"This big van just came and went all over us," witness Abdulrahman Saleh Alamoudi told BuzzFeed News. "He was screaming... 'I'm going to kill all Muslims.'"
The UK's largest Muslim umbrella body, the Muslim Council of Britain (MCB), said on Twitter that the crash happened outside the Muslim Welfare House, just down the road from Finsbury Park Mosque. "Our prayers are with the victims," it said.
At least two witnesses told Sky News there were three men in the van, but the police statement seems to contradict that, saying no other suspects had been identified or reported to police.
Harun Khan, the head of the MCB, described the incident as the "most violent manifestation to date" of Islamophobia, and called on authorities to do more "to tackle the growth in hate crime."