All five members of the pop group, who shot to global fame in the mid-1990s, were spotted clasping microphones as they stood on top of traditional London taxis during the practice session.
Other stars photographed during the rehearsals at the Ford car plant in Dagenham, east London, were Brain May, the Pet Shop Boys, Jessie J, Annie Lennox, Tinie Tempah and George Michael, who has already revealed he is to perform.
The composer, who has written music for Bond films and the hit BBC show Sherlock, added that it would be based around 30 classic pieces of British music.
Take That have been widely tipped to appear, although singer Gary Barlow's presence is in doubt after his daughter Poppy was stillborn.
The ceremony features a cast of thousands, the majority of them volunteers, who have rehearsed during evening sessions.
Arnold said: "We're trying to have moments where someone from the Cotswolds watching it on TV and someone from a tower block overlooking it in the East End will be able to find something in it of which they can say, 'That's us, really that's Britain'."
He told the Telegraph: "To me, this should be the greatest after-party in the world."
"If the Opening Ceremony was the wedding, then we’re the wedding reception. We’re the one where everyone gets out of their cars at the village hall, goes 'Wasn’t that lovely, everyone looked great, let’s put on Blame It On the Boogie and have a laugh.’”