Detectives arrested the man – the fifth suspect – yesterday at a terrace house in Derby under the Terrorism Act 2000. All five are being questioned.

Counter-terrorist officers dressed in white boiler suits and wearing face masks started to search the house in the Normanton district of the city where the 29-year-old man lived.

The search, part of an ongoing investigation by Greater Manchester Police Counter Terrorism Unit and Lancashire Police, is expected to take several days.

Three men, aged 21, 22, and 23 were arrested at Manchester Airport and Accrington on August 14, while a fourth man, 24, was detained in Blackburn on Tuesday morning. He is understood to be a white Muslim convert.

Police sources said officers were investigating threats to kill the Prime Minister, Gordon Brown. It is understood no actual plot was in place but that officers discovered a written threat on an extremist jihadi website earlier this year.

Councillor Fareed Hussain, a member of the Derby Muslim Forum and the city's council, has appealed for calm.

He said: "The community reaction is going to be that of surprise but, from what I know of the details, he is not known to have any connection with the Muslim community in Derby.

"I don't know his name, we haven't got that far, other than the fact that he is Eastern European. He is not Asian or an Arab.

"It's down to the community to pull together as a result of what has happened and to work with each other to make sure that any extremists are weeded out of the community."

Officers stood guard outside the house where the man was arrested. A police officer also watched over the property's garden, which had been paved over.

A 60-year-old neighbour said the man lived at the address with his wife and three children.

The neighbour said: "I just saw lots of police. I asked the plain-clothes officers whether it was the terrorist squad and they said they couldn't tell us anything.

"It's a couple who live there and they have three kids. They are very nice and they stop to say hello."

One neighbour in Moore Street, Normanton, said: "The first time I saw the police was at about 9am yesterday."

The 30-year-old woman, who did not want to be named, added: "I didn't see any armed police, I just saw them sitting there in their vehicles. It's shocking for people in the area."