Detectives are questioning a man under the Terrorism Act after his arrest in a quiet country village.
The suspect, who has not been named, was detained in Hawkinge, near Folkestone, Kent, at about 8.45am on Monday.
The man is being questioned at a police station in the county on suspicion of being engaged in the commission, preparation or instigation of an act of terrorism, a Kent Police spokesman said.
There is no suggestion that the arrest is linked to last week's Channel Tunnel fire or recent investigations in the north of England into an alleged threat to kill Gordon Brown.
Scotland Yard said the Metropolitan Police Counter Terrorism Command was not involved in the inquiry.
Inquiries were being focused on a house in Curlew Place, Hawkinge, a quiet cul-de-sac on a modern housing estate which was cordoned off by police tape on Monday as two officers stood guard.
Neighbours of the arrested man said they did not know him well but believed he was Turkish and lived in the detached house with his family.
Cyril Halpenny, who lives opposite the entrance to Curlew Place, said: "They seemed a quietish family. I only knew them to say hello to, to pass the time of day.
"There was a chap, his wife, and another lady. I think he had a kebab shop or something, I'm not sure."
He added: "There's usually a 4x4 car parked opposite, and it was there this morning, but I believe it's now moved."
