Five men were tonight being questioned by British police after armed officers stormed a suspected bomb factory in a 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.
Detectives said tonight they believe they may have prevented a devastating pipe bomb attack, but it was not clear whether Irish terrorism was involved.
Earlier, a police source said the five men arrested were Irish but a Scotland Yard spokesman said, at this stage, detectives were not linking the arrests to Irish terrorism but were investigating "alleged criminal activity".
In the first raid, at 7.30 a.m., 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.
The second raid, at 3.30 p.m. 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.
In the Southend raid, also at 3.30 p.m., 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.
Specialist officers from Essex Police are expected to continue searching the house in Southend for the next few hours, a force spokeswoman said later.
PA