Big top secret: 2012 Olympic opening ceremony is taking shape in a closely guarded tent

At the Dagenham Ford plant, rehearsals are under way for the showcase

The traffic streaming down the A13 can hardly miss it: a giant blue-and-yellow big top slap bang in the middle of the old Ford factory at Dagenham, east London. But this, in fact, is the world's most top-secret circus.

Police and security guards have been swarming around it like bees since it appeared almost overnight two weeks ago, much to the surprise of local residents, who have not had much to twitch the net curtains over since the last British-built Ford Fiesta drove out the front gates almost a decade ago.

It is under these stripy spires that the Greatest Show on Earth (copyright Seb Coe) is about to take shape. The first full-scale rehearsals for Danny Boyle's Olympics Opening Ceremony are soon to begin. Extra security has been hired to keep away prying eyes and long-lens photographers, but so well-kept was the secret none have turned up.

"I didn't know they were doing the opening ceremony in there," said one female resident of nearby Sierra Drive, separated from the magic by a busy road and a barbed-wire fence.

Another resident, pushing her pram along in the driving rain, said: "My daughter thought it was a circus. The Olympics, is it? What a waste of money."

In December David Cameron doubled the budget for the four ceremonies – opening and closing for the Olympics and the Paralympics – from £40m to £81m, and immediately a call went out for more performers. The 12,000 dancers, drummers, skateboarders, acrobats and lookalikes for British historical figures now secured will be spending an awful lot of time here over the next three months, before the Opening Ceremony at 9pm on Friday 27 July.

Details of quite what to expect remain scarce. Danny Boyle has revealed that the ceremony is named Isles of Wonder and is based on a few tantalising lines from Shakespeare's The Tempest. It has been reported that the actor Mark Rylance, the first artistic director of the Globe and the star of the recent West End and Broadway triumph Jersualem, will read the lines, "Be not afeard; the isle is full of noises, sounds, and sweet airs, that give delight and hurt not."

The same quotation will appear on a huge bell hanging at one end of the stadium, currently being cast at the Whitechapel Bell Foundry, the same company that produced Big Ben and the Liberty Bell in Philadelphia.

If Rylance has been to the Dagenham big top, he has not been spotted. Nor have the locals seen any figures with giant golf balls for heads, as featured in a short video of the rehearsals so far. Women dressed as the famous suffragette Emily Davison, who ran in front of the king's horse, are also expected to appear. Ms Davison would feel particularly at home. It was at the Ford factory in 1968 where women workers went on a famous strike over equal pay, as portrayed in the 2010 film Made in Dagenham.

Hundreds of NHS nurses will also feature in one sequence, and Boyle has promised to capture the British "sense of humour". Pupils from schools in Barking & Dagenham are also taking part.

Other rehearsals will be held at Three Mills Studios, in the shadow of the Olympic Stadium, where Boyle and the director of ceremonies, Billy Elliot director Stephen Daldry, have been working on the events since last year.

The closing ceremony is to be called "A Symphony of British Music", featuring British music "from Elgar to Adele", according to the ceremony's director, Kim Gavin, the man behind the recent phenomenally successful Take That arena tours. Adele, Sir Paul McCartney, Elton John, the Rolling Stones and the Spice Girls have all been tipped to perform. It was recently revealed that the Sex Pistols had declined an invitation to take part.