MI5 agents and anti-terrorist officers were questioning eight young Britons last night after the discovery of the ingredients for a half-tonne fertiliser bomb in a storage unit in West London. The bomb would have been five times the size of the devices used in the al-Qaeda attack on Bali, which claimed more than 200 lives.

Seven of the men arrested are 22 and under, including a 17-year-old student who was seized at an address in Slough. The other man is 32 years old.

The police, who believe an al-Qaeda inspired operation is by far the most likely explanation for the intended attack, fear that terrorists were intending to kill hundreds of civilians with an attack on a “soft target” such as a shopping centre. Only last week Sir John Stevens, the Commissioner of the Metropolitan Police, said that terrorists would target crowded bars, nightclubs, pubs and shopping centres in an attempt to cause mass casualties. He was criticised by David Blunkett for saying that an attack was “inevitable”.

Sir John did not mention that MI5 and five forces were involved in Operation Crevice, which culminated in yesterday’s arrests.

Mr Blunkett praised what he called a first-class police and security operation. In a statement yesterday, he said: “I would like to record the Government’s thanks to all those from the police and security services who work so tirelessly and bravely on our behalf.”

At dawn yesterday 700 officers raided 24 homes and businesses across London and the South East. Police marksmen were on standby as arrest teams and search units raided properties in Ilford, East London, and Uxbridge, Colindale and Hanwell in West London. Other addresses around the M25 were in Luton, Crawley in West Sussex, Horley in Surrey, Slough and Reading. Experts say that the explosive was the same type used by al-Qaeda sympathisers last November in their attacks against targets, including the British Consulate, in Istanbul. Police did not identify the suspected target for the bombers here, but a security source said they are confident that they have the ringleader of this plot among those being held in custody.

With the Easter holidays approaching, security will be tight at many civilian targets including football grounds and shopping centres. Security arrangements for this weekend’s Grand National and other sporting events are certain to be reviewed.

All those in custody in the high-security wing at Paddington Green police station are British citizens, and the majority are of Pakistani origin. The biggest discovery in the raids was at the Access storage company in Boston Road, Hanwell, where police found the ammonium nitrate. The fertiliser could easily have been bought on the internet and would have cost about £60.

The supplier of the chemicals has been traced but detectives are concerned at the lack of effective control on the sale of a chemical that is used to make military explosive.

This same mixture has been used regularly by al-Qaeda groups since their 1998 lorry bomb attacks on two US embassies in East Africa and in the bombings of residential compounds in Saudi Arabia, where Western workers live.

The storage unit is a short drive from Heathrow, and at least three of those arrested live close to Gatwick and addresses were raided near Luton airport. Police do not believe that any of the three airports were the intended target.

The man leading yesterday’s operation, Deputy Assistant Commissioner Peter Clarke, the head of the Anti-Terrorist Branch, took the unusual step of making a public statement within a few hours of the arrests in an attempt to calm public concerns.

He said the operation was part of “continuing and extensive inquiries by police and the Security Service into alleged international terrorist activity and I must stress that the threat from terrorism remains very real. The public must remain watchful and alert.”

More arrests are expected. The police said they were in contact with Muslim leaders in the areas raided to brief them about the reason for the arrests and to assure them that this was not an attack on the Islamic communities living there.

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