A secret act has been lined up to fill the empty slot ahead of headliner Florence And The Machine tonight, Glastonbury Festival organisers revealed, as the main stages opened for the first day of music.

The supporting slot had been left vacant after a last-minute schedule change which saw the band promoted to the coveted top spot when the Foo Fighters were forced to pull out last week.

Swathes of crowds gathered in front of the main stages to watch them launch in to action for the first time this morning, with 90s indie-pop band The Charlatans revealed as the surprise opening act on the Other Stage.

The 135,000 campers have enjoyed sunshine and warm weather since the gates opened on Wednesday but today has seen brief and light showers across the Somerset site.

Singer-turned-activist Charlotte Church, who has been in the anti-austerity movement, interviewed punk rock protesters Pussy Riot on top of a Army-style truck in front of a crowd of thousands.

After overpowering a mock Russian soldier representating Vladimir Putin's administration, Russian feminists Nadya Tolokonnikova and Masha Alyokhina explained the history of their movement, their time in prison and urged the crowds to support Ukraine.

Church said: "It was awesome, they are totally, unbelievably inspiring. They inspire a generation."

Other star acts set to perform today are Mark Ronson, The Proclaimers, Rudimental and Mary J Blige before Florence And The Machine steps in for a performance on the famous Pyramid stage.

Speaking in The Guardian today, Florence Welch made a light-hearted reference to Foo Fighter lead singer Dave Grohl's leg injury and her own troubled love life.

"It's the broken hearts and broken limbs that led to Glastonbury," she said. "It's a quite strange and quite cracked way to get there. Literally. If I'd had six months knowing that I was going to do this (headline slot), I think I would slowly have descended in to madness."

Glastonbury organisers have also confirmed that Azealia Banks has cancelled her performance tomorrow afternoon and has been replaced by The Strypes, while today Bastille filled a "to be announced" slot.