Police say the driver of a van that hit pedestrians on a London road is a 48-year-old man who has been arrested and taken to a hospital.
Mr Osborne was arrested on suspicion of attempted murder and later further arrested over alleged terror offences. Police are carrying out searches at an address in the Cardiff area.
A leader of the Muslim Council of Britain has called for extra security at mosques after a van struck worshippers leaving prayers at the Finsbury Park mosque.
"He has assured me that his team is fully engaged and working in partnership with the London Metropolitan Police as the investigation into the abhorrent attack at Finsbury Park gets under way", Mr Cairns said.
Meanwhile, May said in remarks outside her office in Downing Street that the attacker acted alone and that police responded within eight minutes.
"Over the past weeks and months, Muslims have endured many incidents of Islamophobia and this is the most violent manifestation to date".
We treat this as a terrorist attack. He was overpowered by bystanders and detained until police arrived. A witness told Sky News that two other people fled the scene of the attack, but police said there were no other suspects at present.
Police said the arrested man would be the "subject of a mental health assessment in due course".
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The imam of Muslim Welfare House - which is also a community centre - said a passing police van was flagged down after the attack.
Mohammed Shafiq of the Ramadhan Foundation, a Muslim organization, said that based on eyewitness reports, it seems to be a "deliberate attack against innocent Muslims". It has not been associated with radical views for more than a decade.
Mohammed Shafiq of the Ramadhan Foundation, a Muslim organisation which speaks out against extremism, said: "I utterly condemn the senseless and evil van attack against Muslim worshippers outside the Finsbury Park Mosque in London". The mosque called it a "callous terrorist attack".
The incident comes at a time when emotions are high in England, where there have been several recent terrorist attacks.
Earlier this month, a van veered into pedestrians on London Bridge, setting off vehicle and knife attacks that killed eight people and wounded many others on the bridge and in the nearby Borough Market area.
Prime Minister Theresa May said she would chair an emergency security Cabinet session later Monday.
Sunderland City Council leader Paul Watson said: "Our thoughts and prayers go out to all those caught up in the attack near Finsbury Park mosque". In late May, a suicide bomber killed more than 20 in Manchester at the end of an Ariana Grande concert.