Counter-terrorism officers have been granted a week to question five suspects held in a series of raids in Luton, police said today.
The men, aged between 21 and 35, were taken to a central London police station after officers swooped on their homes on Tuesday.
A warrant of further detention was issued yesterday, Scotland Yard said.
The arrests were a part of a "pre-planned, intelligence-led" operation by the Metropolitan Police counter-terrorism command.
The men, aged 21, 23, 24, 30 and 35, are being held on suspicion of the commission, preparation or instigation of acts of terrorism.
Searches under the Terrorism Act 2000, which took place at each of their addresses, have now concluded.
A Scotland Yard spokesman 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.
"On April 25 a warrant of further detention was issued, to expire seven days after their arrest."
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