By Ian Youngs
Entertainment reporter, BBC News at the Glastonbury Festival
Amy Winehouse has kicked off the action on the main stage at the Glastonbury Festival, as more rain threatens to turn the event into a mudbath.
Winehouse sang songs from her album Back to Black
Steady rainfall has transformed the site's main pathways into a couple of inches of mud.
Hundreds of tents have begun to submerge, and more showers are forecast across the weekend.
Winehouse - followed by The Magic Numbers - was the first act to perform on the main Pyramid stage.
A brief glimpse of sunshine during her mid-afternoon set led to thousands of revellers roaring in approval, but the wet weather soon returned.
Rhys Evans, 29, from Cardiff, was one of scores of people whose tents were affected by mud.
He said: "There's a couple of inches of horrible, brown mud in my tent. I'm going to have to drink a hell of a lot for it to go away."
The Killers, Bjork, Dame Shirley Bassey and The Chemical Brothers are among the other big names performing at the three-day festival.
30,000 cars in car park
13 thefts from tents
12 people tried jumping over the fence
Tight security greeted the last ticket-holders as they took their places in the event's vast tent city.
The campsites are packed after the capacity was raised to 177,500 - 27,500 more than the last festival in 2005.
Photo tickets, sniffer dogs, CCTV and bag searches are among the measures being taken to beat touts and crime.
After successfully stamping out gatecrashers, police and festival security have now turned their attention to drugs.
Sniffer dogs have been deployed at train stations and on coaches, while random bag searches are taking place at festival gates.
Some people have found unusual ways to get around the site
Police have also laid traps for tent thieves, erecting "covert capture tents" equipped with cameras.
Fans who arrived early have had the chance to check out the festival's major new addition, an area called The Park.
It boasts music stages, a tipi village, a silent disco and a 12-metre viewing tower with telescopes.
Another new feature is an installation by cult artist Banksy, a sculpture made out of portable toilets arranged in the manner of an ancient stone circle.
Fittingly for a work by Banksy - who made his name as a graffiti artist - revellers have already spray-painted all over the new creation.
Glastonbury is also known for its environmental conscience and fundraising, and Thursday also saw the launch of the I Count climate change campaign.
The Arctic Monkeys will headline the Pyramid Stage on Friday
Festival founder Michael Eavis launched the drive to get 100,000 people to sign up to the commitment to reduce their carbon footprints.
Laura Fishwick, 19, from Leeds, gave the festival the thumbs-up on her first visit. "This has totally blown me away," she said.
"I thought it would be smaller and more wild, but everyone's really friendly and [it is] really safe," she said.
Twenty-four-year-old Angus Vine said: "Even at this stage, you can tell it's more advanced than any other festival around.
"It's very well managed. The weather's the only thing they can't control."