A man rammed a van into worshippers coming out of a mosque in north london after special Ramzan prayers on Monday, killing one person and injuring 10 others.
The attack - the fourth terror assault to jolt Britain in four months — occurred just after midnight outside the Muslim Welfare House, which houses a mosque, on the Seven Sisters Road when some people were helping a man who had collapsed.
The site of the incident is close to another mosque near the Finsbury Park on the same road.
One man was pronounced dead at the scene and eight others were injured. Two others were treated for minor injuries.
A 48-year-old man, the driver of the van, was detained by members of public at the scene and then arrested by police on suspicion of attempted murder.
"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," May said in a statement after chairing a meeting of the government's emergency COBRA committee.
In an attempt to counter some criticism over the delay in classifying the incident as a terrorist attack, she said: "Officers were in the immediate vicinity as the attack unfolded and responded within one minute. Police declared it a terrorist incident within eight minutes."
Home Secretary Amber Rudd described the attack as an appalling incident, and said new funding for security at religious sites had recently been arranged.
Metropolitan Police deputy assistant commissioner Neil Basu said the man who died was already receiving first aid from members of the public and it was not yet known if his death was caused by the attack.
"This is being treated as a terrorist attack and the Counter Terrorism Command is investigating. At this early stage of investigation, no other suspects at the scene have been identified or reported to police, however the investigation continues," said Basu, london’s senior counter- terrorism coordinator.
"All the victims were Muslim and there are currently no- other suspects," he added.
Eyewitness Abdul Rahman said the driver said he wanted to "kill all Muslims".
"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," Rahman said.