Five men have been arrested in Luton on suspicion of the commission, preparation or instigation of acts of terrorism, Scotland Yard said.
The arrests were part of a "pre-planned, intelligence-led" operation by the Metropolitan Police counter-terrorism command.
The men, aged between 21 and 30, were all arrested at separate homes in Luton this morning and have been taken to a central London police station for questioning.
Searches under the Terrorism Act 2000 are taking place at all five of the addresses and inquiries are ongoing, Scotland Yard said.
A spokesman said: "Officers from the counter-terrorism command have today, Tuesday April 24, arrested five men on suspicion of the commission, preparation or instigation of acts of terrorism.
"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.
"The arrests were a part of a pre-planned, intelligence-led operation.
"Search warrants were also executed under the Terrorism Act 2000 at five residential addresses in Luton in connection with this inquiry and searches are ongoing."
Officers stood guard outside an address on Bishopscote Road at lunchtime as police examined the inside of a semi-detached home.
Others living on the same street spoke of their shock and described their neighbours as "friendly" people.
Self-employed Mohammed Iqbal, who did not give his age, said he believed the man arrested on Bishopscote Road lived with his family.
"I see him passing by sometimes," he said.
"I don't know him but he seems friendly.
"I think his family are all grown-up."
Another neighbour living next door, who would not give her name, added: "They are very nice. They have been very good neighbours."