Counter terrorism police smashed through the door of a St Albans house in an early morning raid last week.
Neighbours living in Francis Avenue said Metropolitan police showed up “unexpectedly” just before 6am on Wednesday.
The London force, which deals with counter-terrorism cases aided by Hertfordshire Police officers, remained at the property for the rest of the day.
The house was was one of 12 addresses searched as part of a wider Metropolitan Police investigation into suspected terrorist activity in Syria.
One neighbour said he was woken up by loud banging noises that startled him out of bed to see his street “flooded with police cars”.
He said: “The loud banging, as they smashed through the door of the house, woke us up.
“It was pretty early- just before 6am. It is shocking because ever since we have lived here, it has been a quiet street where not much goes on.
“You don’t expect to be awoken to look out the window see your street flooded with police officers.”
This week the house remains boarded up.
The other searches have taken place in west London. A 32-year-old man from west London was arrested on Wednesday morning on suspicion of terrorism offences.
Another neighbour said: “I have lived in St Albans for many years now and you don’t hear of things like this going on, let alone on your street.
“I asked the police what was going on, but they said they couldn’t assist. When I heard it was terrorism- I was shocked.
“It is very concerning. It makes me feel uncomfortable living here.”
No one has been arrested.
In a separate incident last month, a Watford address was searched and a 44-year-old man was arrested under the Terrorism Act 2006.
Tahir Bhatti from Clarke Way, Watford, was charged with assisting a 26-year-old man from London to commit terrorist acts.