The police were ready, the stones switched. Then a JCB smashed into the Money Zone...

With a plot worthy of a James Bond script, a starring role for the Millennium Dome and a prize of £350m in diamonds, it would have been one of the all-time great heists.

With a plot worthy of a James Bond script, a starring role for the Millennium Dome and a prize of £350m in diamonds, it would have been one of the all-time great heists.

Yesterday's attempted robbery at the Dome even shared a scene with the Bond film The World Is Not Enough - a power boat getaway along the Thames.

But months of undercover work by Scotland Yard's Flying Squad and a carefully planned ambush turned a potential disaster into a triumph for police intelligence.

The team of old-style villains never stood a chance. From the moment their digger smashed into the Money Zone under the cover of smoke bombs and they began hammering at the reinforced cabinet that contained what they thought were 12 ravishing gems they were doomed.

The robbers had been under surveillance for months, and officers from the Flying Squad, better known as "The Sweeney", had been expecting the raid for weeks, although they discovered the timing only the night before. So De Beers, which owns the jewels, promptly switched them with fakes, in case anything went wrong.

Members of the gang had been tracked and bugged for months, after being linked to two failed robberies in Kent and London this year.

Yesterday's raid started just before 9.25am when four robbers disguised as workmen arrived at the perimeter fence close to the main entrance. One was driving a stolen digger, with its heavy bucket upside down to protect himself.

At 9.30am the digger crashed through the perimeter fence and into the Dome through a large plastic panel, leaving a trail of twisted wreckage.

Moments later, a man moored a high-powered speed boat at the Millennium Pier close to the Dome, as the mechanical digger was driven through the Learning Zone and towards the Money Zone within the inner circle of exhibits.

Outside the target area, four balaclava-masked men donned gas masks, detonated smoke bombs and released ammonia gas to deter interference.

At 9.35 they burst into the Money Zone and started using hammers, sledgehammers and a nail-gun to smash the reinforced, bullet-proof glass cylinder protecting the gems.

Inside the cylinder lay what the gang thought were 12 diamonds, including the 203-carat (40-gram) Millennium Star, which had been at the centre of a Millennium ceremony with Sophie Marceau, the Bond girl in The World Is Not Enough.

But once the four men were inside the Money Zone, police moved in. Alongside the 200 staff on duty at the Dome were dozens of armed officers dressed as cleaners, their Heckler & Koch semi-automatics in black plastic binliners.

The whole operation involved 100 police from the Flying Squad, the special firearms unit and the Kent force. At 9.40am the officers surrounded the robbers, frantically trying to smash the display case, and told the men to come out with their hands up.

Detective Superintendent John Shatford of the Flying Squad said: "As the robbers went into the vaults they effectively imprisoned themselves and were ordered to come out at gunpoint."

A Millennium Dome guide, who refused to be named, said only when he saw men wearing balaclavas and the JCB he realised something was wrong. "I thought 'What the hell?' It was coming straight for us. It came so near us we had to jump out of the way. Then it turned and headed straight for the door and I thought 'Hold on a second, mate. The door isn't open yet. You can't just drive straight through it'. But it did.

"There was no one about, which was strange. They quickly got out of the digger and ran into the Money Zone. Then it seemed like armed policemen came out of nowhere and pulled us back. They kept shouting 'Keep back! Keep back!'"

Ozcan Ocak, a worker at the Dome, added: "I heard a terrible crashing noise as if there was an accident. When I went out to look I saw a bulldozer outside going through the Money Zone. There were police everywhere. It was like something out of a movie. Someone next to me even asked whether this was part of the live entertainment at the Dome."

Four men were arrested and a fifth was held with a car on the north side of the Thames near Bow Creek where he was said to be monitoring police radio frequencies with a scanner. There were no shots fired and no guns were found.

Another man was arrested on a power boat at a Dome pier on the Thames. Kent officers raided two addresses in the villages of Collier Street and Horsmonden, and arrested six more men.

Det Supt Shatford said: "The escape on the boat was clearly audacious. Clearly that was to be used to make their escape across the river."

Some 60 visitors were in the Dome at the time of the robbery attempt. Police had managed to keep them away from the Money Zone without blowing their cover.

A school party of up to 100 children from All Hallows School, in Farnham, Surrey, were listening to a lecture in the Mind Zone when an official herded them into a small room as the security alert began. They sang hymns until the end of the police operation.

David James, the NMEC executive chairman, said the Dome chief, Pierre-Yves Gerbeau, told him of the threatened raid when he took up his post nine weeks ago. "We had a number of false alarms," he said. "The police didn't know when it was going to happen."

Asked how many false alarms there had been he said. "It was over many weeks. I would say as many as two dozen." The gang behind the robbery, who are not thought to have had any inside help from Dome staff, are believed to be the same men responsible for two equally unsuccessful jobs earlier this year.

In July, a Securicor van was blocked by a lorry and another van shortly after it left its depot at Aylesford, near Maidstone, at 7.30am. Robbers leapt out waving guns and attached explosives to the front and rear of the van. But attempts to use a lorry fitted with a metal spike to prise open the security van door failed and the explosives fell off on to the ground. The gang fled on a speedboat on the nearby river Medway.

In February, armed robbers attacked a Securicor van in London. The five-man gang held up the van as it left the depot in Kirtling Street, Nine Elms, at 6.45am. They set fire to vehicles to block the road and surrounded the van, trying to break into it using bolt-cutters, but they failed and ran away empty-handed.

Yesterday's arrests will be a coup for Scotland Yard and the Flying Squad, whose reputation as the famous "Sweeney" - taken from the Cockney rhyming slang "Sweeney Todd" - took a dive after repeated corruption inquiries.

The Dome's reputation as a place of high thrills has also been enhanced, although the exhibition's boss, Mr Gerbeau, dismissed suggestions that the foiled robbery was the most exciting thing that has happened there since it opened.