Operations took place in Birmingham, London and other areas
Armed British police have raided properties and made several arrests linked to Wednesday's Westminster attack.
Acting Deputy Commissioner at the Metropolitan Police, Mark Rowley, says hundreds of detectives have been working through the night.
Police searched six addresses and made seven arrests.
The operations took place in Birmingham, London and other parts of Britain - and are continuing today.
Deputy Commissioner Rowley said authorities believe the attacker, who has also not been named, acted alone and was inspired by international terrorism.
Sources say a number of people have been arrested at a property in Hagley Road in the Edgbaston area of Birmingham.
A witness who works nearby told the Press Association: "The man from London lived here. They came and arrested three men."
Dozens of officers, some armed, were pictured outside the second floor flat, above a parade of shops, which was cordoned off by police.
Roads around the address have now been re-opened, but officers remain at the scene.
West Midlands Police referred enquiries about the incident to the Met Police, which said it was "not prepared to discuss the matter for operational reasons".