An electrical storm led to the plug being pulled on Glastonbury's Pyramid Stage temporarily.
Lightning streaked through the sky while claps of thunder drowned out music as torrential rain hit festival-goers, leaving organisers saying it was unsafe to continue.
The weather problems led to Lily Allen's set being delayed by 45 minutes, while other stages also suffered setbacks.
The rain eventually stopped and a rainbow appeared in the sky as Glastonbury regular Allen came on, telling the crowd she had last played the Somerset site five years ago.
After bursting on to the stage with Smile, she told the huge crowd: "It was the night I got together with my now-husband. Now he's here with our two beautiful babies. I love you all so much."
Her husband Sam Cooper could be seen watching from the side of the Pyramid Stage with one of their two young daughters on his shoulders.
Wearing a full-length gold skirt and hot pink top which matched her hair, she said: "The rain stopped - thank God for that."
She later returned to the stage having removed the skirt to reveal a crotch-skimming hot pink dress. The crowd booed as Allen said her set had been cut short due to the weather-related delays to make way for Elbow and headliners Arcade Fire later on.
Earlier the storm led to Rudimental's set being cut short and festival-goers running for cover from the heavy rain, which followed an afternoon of sunshine.
But some appeared to enjoy the muddy conditions by having a splash around. Among them was seven-year-old Coral Bickwell, who appeared to delight in getting as much as she could over her until she was covered from head to toe.
Singer and activist Billy Bragg, who believes he has performed at the festival around 20 times, said revellers should wear the mud "with pride". "Don't mind the mud. It's not any mud, it's Glastonbury mud," he said.
Bragg, who curates the festival's Left Field, said Glastonbury summed up what England is all about. "When I think of those words from Blake's Jerusalem, this green and pleasant land, I think of this place and these people together and this is my idea of what England is all about - you know, rain, weak tea, smelly toilets and great people having a great time.
"That's the one thing you must always bring to Glastonbury with you every year, is a British sense of humour. When I was trying to find my breakfast this morning at 9 o'clock in the pouring rain with my hood up, plunging around in the mud, one of the bars over there was blasting out Walking On Sunshine.
"There's your first Glastonbury moment of the weekend already."
Festival organiser Michael Eavis promised there would be finer weather ahead. "I'm sorry about the rain, but it's going to get better," he told the Left Field tent. "Sunday is going to be a fantastically sunny day."
Although the rain - expected to continue tomorrow - was not welcome to many, stall-holders charging £15 a time for wellies and waterproofs were enjoying a roaring trade.
Proceedings kicked off on the Pyramid Stage today with 17-piece Japanese orchestra Turtle Island, who made an enthusiastic effort to banish the bad weather.
Acts on other stages during the first official day of music included Blondie, Haim and Kaiser Chiefs while supermodel Jerry Hall brought an injection of glamour as she performed with her new band Paris-Texas And The Uke.
This afternoon the Greenpeace field was visited by Dame Vivienne Westwood who appeared as a guest speaker on fracking and climate change.
The fashion designer turned environmental campaigner said this was her first time at Glastonbury and thought it was "lovely".
Among the other famous faces seen today was singer Rita Ora who turned heads in her black and white cow-print dungarees, perhaps inspired by the animals of Worthy Farm. Radio One DJ Fearne Cotton wandered the site in an eye-catching multi-coloured jacket and an essential pair of wellies, while Queens Park Rangers footballer Joey Barton was also spotted.
Police said 85 crimes were reported within the first 48 hours of the festival opening its gates on Wednesday morning, a 30% decrease on last year. About half of these were property thefts, with 30 people arrested so far. Police said they used tracking software to locate one stolen phone and arrested a woman overnight.