The organisers of Boardmasters Festival say they are "disappointed" by the number of tents left behind at Watergate Bay.
As revellers started leaving the Newquay site on Monday 14 August, a layer of rubbish remained on the fields usually covered by sheep.
Jacqueline Jones, 79, who lives opposite the site, said she looked forward to the festival every summer but "felt sad" to see the mess left behind.
"I get quite excited when I see all the kids arriving. The site is so beautiful, it’s like something from fairyland, or Aladdin and the Arabian Nights.
"But why do they have to spoil it by leaving all this rubbish behind? I just don’t understand the mentality.
"It’s like an ocean of plastic and so - as you can imagine - all the seagulls are having a ball. It looks like a bomb site."
The mess is despite the "litter bond" scheme offered by Boardmasters, which encourages revellers to collect and return a bag of rubbish in order to redeem a £10 deposit included the overall cost of their ticket.
The festival also works with Help For Homeless Cornwall to salvage any good-quality tents left behind on the campsites.
A team of litter-picking volunteers will now ensure the site can be safely used by farm animals again by the end of this week.
A Boardmasters spokesman said: "Festival-goers have been leaving Boardmasters campsites today and we expect all to have left by 4pm.
"As festival-goers have been packing up, our green team volunteers have been stationed in the campsite, handing out bin bags and encouraging customers to do their bit to tidy up.
"We are disappointed to see that some tents have been left behind, following extensive signage and messaging across the sites asking attendees to do their bit and Leave Only Footprints.
"Our ‘Leave Only Footprints’ green pledge outlines all of our efforts for not just us, but also for our attendees and Boardmasters family.
"As part of the robust clean up operation, the Green Team has also been down at Watergate Bay to clean the beach and local area.
"The Litter Bond scheme has remained open until 2pm, for festival-goers to convert their rubbish to cash, while the manned campsite information point are collecting unused food for Cornwall Food Aid.
"Cleaning is underway on walking routes to Newquay from Watergate Bay and litter picking is underway in the campsites.
"Litter picking continues tomorrow, focusing on the campsites closest to Watergate Bay Road.
"Plus, we’ll be litter picking in the local area around the festival site and removing bigger bits of rubbish.
"Wednesday and Thursday will focus on clearing the rest of the campsites and the main arena. We’ll also be picking up the smaller bits of rubbish.
"By Friday, the fields will be cleared and ready for the sheep to come back."