Scotland Yard on Tuesday arrested five men on suspicion of terror offences in sweeping raids in the town of Luton, northwest of London.
The men, aged 21, 23, 24, 25 and 30, were arrested at separate homes in Luton "on suspicion of the commission, preparation or instigation of acts of terrorism," the police said.
They said the raids were "intelligence-led", and were conducted at separate addresses.
A Scotland Yard statement said, "All five were arrested at separate residential addresses in Luton. They have been taken to a central London police station where they remain in custody."
"This morning's arrests, at five different houses in the Bury Park area, are part of a pre-planned, intelligence led operation by the Metropolitan Police Counter Terrorism Command," Bedfordshire Police said in a separate statement.
The Bury Park area is home to a large Pakistani community.
It added search warrants were also executed under the Terrorism Act 2000 at five residential addresses in Luton in connection with this inquiry and searches were ongoing.
"At the moment, officers from the Metropolitan Police are carrying out thorough searches of the addresses of the people who have been arrested and this is expected to take a minimum of 24 hours," Bedfordshire police said.
Families of those people who have been arrested have been given the option of finding alternative places to stay, if they prefer, but there is no requirement for them to leave their homes should they wish to remain while searches are conducted, the police said.