Festival-goers ran for shelter and donned their waterproofs as rain hit Glastonbury.
A succession of short bursts of heavy showers sent music fans into tents and marquees as they sought cover after enjoying warm sunshine this morning.
Earlier police warned of the dangers of taking illegal drugs after a 26-year-old man was left fighting for his life in hospital after taking ketamine at the site.
An Avon and Somerset Police spokeswoman said the man, from the south east of England, is currently in a "life-threatening condition" at Bristol Royal Infirmary.
His family has been informed.
Today, more people were trickling onto the site, which was opened to the public yesterday morning.
Many were already wearing their wellies when the rain started just before 2.30pm, clearly prepared for the wet weather that was forecast.
As the heavens opened, tents hosting acts that had previously attracted small crowds quickly filled up while the site's walkways became a sea of colour as people donned raincoats and ponchos of every description.
Others created makeshift shelters from whatever came to hand, while some rocked out colourful headgear with small umbrellas attached to the top.
The ground remained hard underfoot but forecasters warned that with the rain levels predicted it may not remain this way for long and could see the site resembling one of its legendary mud bath years.
Heavy showers are expected to continue for the rest of the day while the Met Office has issued a yellow weather warning for rain for tomorrow and Saturday for the southern half of the UK, including the Glastonbury site.
An area of low pressure is bringing unstable weather with the heaviest downpours potentially bringing 15-20mm of rain in less than an hour while there could also be thunder, lightning and hail.
Police said crime at the festival was 50% down when compared to this time last year with arrests for drugs-related offences and thefts, with fewer reports of mobile phones being stolen this year but an increase in cash being taken.
But they warned festival-goers to keep a close eye on their valuables with today usually being the busiest day for crime.
Police said it is not believed the batch of Class B substance ketamine the man in hospital had was contaminated, but that he suffered an adverse reaction on taking it. A 67-year-old woman also died at the site last night from natural causes.
Her death is not being treated as suspicious.
Tomorrow's first official day of music will see Elbow, Lily Allen and Rudimental perform on the Pyramid Stage ahead of headliners Arcade Fire, while other acts across the site include Haim, MIA, Jurassic 5, Metronomy and Kaiser Chiefs
The rain eased off this evening and although walkways were left mucky, there was no serious mud so far.
Hundreds of thousands of pounds have been spent on improving the site's drainage systems over the years but with so many people trampling the fields, it does not take long for the ground to get churned up.
Organisers of the first ever British Mud Week, which was set up to encourage people to embrace the outdoors, have timed it to coincide with Glastonbury.
Damian Lockley said: "Messing about in the mud can be good fun. Everyone's got their wellies, the music's playing, the beer's flowing, there's nothing you can do about it so just enjoy it.
"The England football team and cricket team might not have shown much fight just lately but Glastonbury is a chance for people to show that good old British bulldog spirit is alive and well."
The areas in front of the main stages remain cordoned off to keep the grass intact until tomorrow.
Sound checks were being carried out tonight, with loud music blaring from the Pyramid Stage, giving festival-goers a taste of things to come.