Festival-goers had to sleep in their cars after traffic became gridlocked when heavy rain turned the Isle of Wight Festival site into a mudbath.
The mud led to access problems at Seaclose Park in Newport on Thursday night leaving thousands unable to get into car parks.
The Isle of Wight Council said traffic was clearing on Friday afternoon, with small delays to get into the site.
About 55,000 people are expected at the three-day event.
The Isle of Wight has a population of just 140,000.
The problems came as various parts of the UK were braced for heavy rain and flooding. The Met Office has issued yellow warnings for various parts of England and Scotland and a more severe amber warning for the North West of England.
Met Office forecasters said the island would see very windy weather, with sunny spells and scattered blustery showers over the weekend.
Motorists queued for up to 10 hours to access the site.
Linda Dawson, from Ashurst, said she had been stuck in her car with her partner and three daughters aged 12, 13 and 16 since 18:00 BST on Thursday.
She said: "The two things I'm really annoyed about is this weather has been going on for weeks, so why were they not prepared for it? Second we weren't told the full story in Portsmouth and could have turned back if we had known we would be in the car all night.
"Why did they let us get on the ferry when they knew there was nowhere for us to go?"
Festival-goers have taken to social networking sites to complain about their treatment and a lack of communication from festival organisers. Some said they had given up and gone home.
Jo Wendel tweeted the message: "Awful! Would only recommend #iowfest to lovers of disorganisation, queuing and mudbaths."
Simon Perry, co-founder of site VentnorBlog , which covers the island, said: "Many readers questioned why the relevant authorities hadn't planned for this, given the unseasonable recent heavy rainfall."
Problems began on Thursday morning when the main car park had become waterlogged from heavy rain overnight.
Organisers were forced to open extra fields and bring in metal tracks so vehicles could be directed to overflow car parks. Many had to be towed because of the mud.
Festival organiser John Giddings said: "We did everything within our power but as soon as that 24 hour rain came down it became horrendous.
"I'm really very sorry, I can only apologise, I know there's a couple of hundred still out there.
"It's going to be a great weekend, don't give up on us."
On Friday morning organisers urged anyone still waiting to get to the island to leave their cars and travel on the ferries as foot passengers.
Mr Giddings said plans were also being put in place for people leaving on Sunday.
He said: "The police want an emergency plan from me as to how I am going to get people off on Sunday.
"I'll have tractors and people ready to take people off."
Local resident Lynette Howes from Ryde said her son had not taken his children to school in East Cowes after waiting for a bus for three hours to get home from the school run on Thursday morning.
She said: "He gave up waiting went back to the school, took them out of school and walked.
"It's always been well organised, it's a good thing for the island but we've never had this chaos before, I wouldn't like to see the festival go though."
Andrew Turner, MP for the Isle of Wight, called on the council to investigate the problems and review its decision to increase capacity last year.
He said: "It appears that inadequate contingency plans and preparations were in place.
"Lessons must be learned. Most people are not killjoys - they expect some inconvenience over the festival weekend - but there must be a limit. That limit has been reached.
"The permitted festival size was increased to 90,000 this year - that is too many and not all the tickets have been sold. If they had been the problems would be even worse."
An adult weekend camping ticket for the festival, which runs from Friday until Sunday, is £190 and headliners include Tom Petty and the Heartbreakers, Pearl Jam and Bruce Springsteen.
The date of the event was moved from its normal slot in early June to avoid clashing with the Diamond Jubilee and half-term holidays.