A heavy shower on Friday afternoon brought out the wellies, ponchos and umbrellas and created puddles on site.
Friday's acts include Florence and the Machine, Motorhead, Mark Ronson and an unconfirmed band, rumoured to be The Libertines, on the Pyramid Stage.
However, Professor Stephen Hawking is unlikely to attend the event for an appearance in the Kidz Field.
A spokesperson for the physicist told The Telegraph he had pulled out "for personal reasons".
In other Glastonbury news:
Organisers have not confirmed the identity of the act who will fill the Pyramid Stage slot on Friday that was left vacant when Florence Welch was promoted to the headline slot.
Rumours about Pete Doherty and Carl Barat's band The Libertines circulated after the name "Albion" was pictured on a backstage running order. The Libertines have used the word Albion frequently during their careers.
Florence has replaced Foo Fighters, who pulled out when frontman Dave Grohl broke his leg.
The decision to promote Florence and the Machine to headline status was criticised by some fans, but organiser Emily Eavis said she made the decision "straight away".
"This is her moment," Eavis said. "She's had a number one record in America, a number one record here, and she's on fire."
'Culture of rebellion'
Meanwhile, feminist punk band Pussy Riot made an appearance in front of the Park Stage, using a theatrical protest to convey their anti-government message.
The appearance began with an actor posing as a Russian soldier standing atop a military van and declaring Glastonbury a pro-Russian republic.
Pussy Riot's Nadezhda Tolokonnikova and Maria Alyokhina, who were imprisoned in Russia for almost two years for their protests, then climbed up, tied him up and put one of their trademark multi-coloured balaclavas on him.
Tolokonnikova told the crowd to "develop a culture of rebellion" and, in a remark possibly aimed at other bands, said they could not just "sit on a comfy coach and drink some beer".
They were then interviewed on top of the van by singer Charlotte Church, who described them as "one of the most important movements this century".
A total of 177,000 people are due on site. The weather is expected to brighten up later, with Saturday expected to be sunny but more rain likely on Sunday.
Rain has fallen on the Glastonbury Festival as fans enjoyed the first day of music on the main stages.