Heavy rain in recent days turned many of the main access routes into a quagmire as the first of 153,000 music fans poured into the site.
But an influx of early arrivals on Tuesday night caused traffic gridlock as thousands of vehicles queued to get onto the site.
Oxfam volunteer Jim Barnes posted on Facebook: “We’ve been queueing in our van for 14 hours and we’re meant to be working for oxfam…. What’s the best way into purple parking?”
The early birds had failed to heed a warning from organiser Michael Eavis to delay their arrival until Wednesday morning.
He said: “We did ask people to come in later but we’ve had lots of early arrivals this year.”
The car parks were due to open by 9pm on Tuesday evening but some revellers claimed they were not able to get in.
Zoe Jones posted on Facebook: “Can you tell us why we haven’t even moved in two hours?
“I wouldn’t mind if we were moving but they’re obviously not letting anyone in from our direction.”
Police and the festival organisers pleaded with ticket-holders to delay their journeys until later on Wednesday.
A 6.30am update from the event said: “If you are coming to the Festival by car or campervan / caravan and have yet to begin your journey, please do not set off yet.
“If you have set off and have yet to reach the site, please stop somewhere safe and warm.
“If you are coming to the festival by coach or train please set off as planned. But be prepared for your journey to take longer than usual.”
A statement added: “The current wet weather and ground conditions are causing heavy traffic congestion around the Glastonbury Festival site.
But when the gates to Worthy Farm finally opened at 8am the revellers were wading through mud and puddles.
Cider Bus’s general manager Nick Laurie said: “We’ve tamed the mud. It was pretty awful.
“When we arrived in the corner of our marquee was a foot deep pond, which was pumped out, but when we brought in the big vehicles like the lorry the mud came up to the axle.”