Five men were tonight being questioned by anti-terrorist police after armed officers stormed a suspected bomb factory in a quiet London suburb, then swooped on two another houses.
Parts that may have been intended to make pipe bombs were seized in the dawn raid in Ealing, west London, where the men were arrested on suspicion of conspiracy to cause explosions.
Later, about 20 armed police searched an address in Fosse Way, a quarter of a mile away, while 50 officers raided a house in Southend, Essex.
Detectives said tonight they believe they may have prevented a devastating pipe bomb attack, but it was not clear whether Irish terrorism was involved.
In the first raid, at 7.30am, anti-terrorist branch detectives and officers from the firearms unit of the Metropolitan Police swooped on a ground floor flat in Sutherland Road, Ealing.
No shots were fired and no one was injured. The five men were taken to a central London police station for questioning.
Police sealed off the street and forensics officers carried out a fingertip search of a ground floor flat.
Chief Superintendent Peter Goulding: ‘‘In the operation we recovered a number of items which are of interest to us. We believe that as a result of this operation we have prevented a potential pipe-bomb attack.’’
The second raid, at 3.30pm at a house in nearby Fosse Way, was accompanied by a search of a nearby field, with more than 20 armed officers combing Ealing Rugby Club with sniffer dogs.
Officers also used rakes to search a ditch surrounding the field, which backs on to Hatherway Primary School and Castlebar Special Needs school.
Residents in adjoining Vallis Way told how officers searched and questioned builders working on the construction of a new stadium at the ground.
In the Southend raid, also at 3.30pm, about 50 police, some armed, descended on a house in York Road. No one was arrested at the house but police said they had earlier arrested a man who had since been released without charge.
Despite earlier speculation, it is understood the operation was not linked to animal rights extremists.
The raids coincided with Ulster Unionist leader David Trimble’s trip to London for urgent talks with Prime Minister Tony Blair in a bid to break the deadlock in the Northern Ireland peace process.
Earlier, a police source said the five men arrested were Irish.
A Scotland Yard spokesman said that, at this stage, detectives were not linking the arrests to terrorism but were investigating ‘‘alleged criminal activity’’.
The spokesman said: "At this stage of the investigation, there is no evidence to suggest that these arrests and seizures are linked to terrorism, nor the three terrorist attacks in London during 2000".
Those attacks, including one on a rail line near the scene of today’s Ealing raids, were believed to be the work of dissident republicans from the breakaway Real IRA.
Arnold Aarons, administrator of the Ealing Liberal Synagogue which is about 300 yards away from the first raid, said: "It’s alarming to learn that there are people living in this vicinity planning terrorism".
Josephine Newe, whose bedroom backs on to the apartment raided by police, said police had arrested a jobless man called John.
Miss Newe, 26, said: "I jumped out of bed. I didn’t know what was going on. I just heard the glass smashing and the police said ‘Stay indoors’.
"I nearly had a heart attack when I heard about the pipe bombs on the news. I didn’t know what was going on."
A resident near the scene of the second London raid, Stephen Jardine, 51, said: "I’ve lived in Ealing for 20 years and I’ve never seen anything like it. They were searching vans as well as builders coming out of a site. They seemed to be searching people".