A man reportedly announced he wanted to "kill Muslims" before driving a van into a crowd in London's north end, striking several pedestrians near a mosque in the United Kingdom city.
They were carrying out searches of addresses in Cardiff where the vehicle hire company that the van was rented from was based.
London's police commander says the van attack near Finsbury Park Mosque was clearly an attack on Muslims.
The suspect "was very quickly and calmly given over to the police", Metropolitan Police Deputy Assistant Commissioner Neil Basu said, as he praised the community for their bravery.
One person died and 10 others were injured in the incident, which British Prime Minister Theresa May has called "sickening", and London's mayor has described as a "horrific terrorist attack".
A 47-year-old man who struck a crowd of worshippers leaving a mosque in Finsbury Park, London, on Monday morning has been arrested on suspicion of commission, preparation or instigation of terrorism including murder as well as attempted murder.
When he woke at about 1 a.m.to the sound of a helicopter circling overhead, Hornak said, his first thought was another terrorist attack.
Later, she visited Finsbury Park Mosque and met people from a variety of faiths.
"Our community is in shock", he said, urging people attending prayers to remain vigilant, a warning that was echoed by police.
Police arrested the van's driver, but were trying to determine whether the collision was accidental or deliberate.
This is the third attack in London since March and the second in which a vehicle was used as a lethal weapon. "I feel scared", Mohamed said.
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.
He was restrained by members of the public at the scene, with an imam from the nearby mosque protecting him from angry members of the public. "By God's grace we managed to surround him and to protect him from any harm", Mahmoud said.
"I have assured the leadership of the mosque and the Muslim welfare centre of our prayers and support", he said. "Our prayers are with the victims", the Muslim Council of Britain (MCB), an umbrella body, said on Twitter.