Up to 50 extra police officers have been sent to the Isle of Wight Festival because weather conditions are expected to worsen.
They will join the 100 police officers and staff on site and will help assist revellers heading home on Sunday.
Hampshire's Chief Constable, Alex Marshall, said the rainfall had had an "unprecedented impact".
Audiences had a two-hour wait before the walkway to the main festival site was opened at 16:00 BST on Friday.
Mr Marshall said: "It was a wet and muddy start for many campers this year but thankfully the weather did not seem to dampen the festival spirit on site.
"I'd ask that those heading home by car drive sensibly and carefully. Due to the volume of traffic on the site drivers should expect some delays and we would ask you to be patient and remain with your vehicles, as the organisers work to get you home.
"The unprecedented impact the rain had on the site brought with it a unique challenge in the festival's 11-year history."
On Friday, performances from the likes of Feeder, Elbow, Lana Del Rey and Tom Petty and the Heartbreakers were subject only to minor delays.
Taka Hirose, bassist with Feeder, said the field was almost empty when his band began their set.
He said: "We were a bit worried, obviously, being on so early, and obviously there was also a problem with the doors opening late.
"So we tried to shorten our set and go on later, but I think they let us have five minutes later, but after three songs it suddenly filled up and by the end it was heaving."
BBC South weather forecaster Alexis Green warned the heavy rain is expected to return, moving in from south-west.
She said: "We are expecting between 15-20mm of rain, around an inch.
"It's not just the rain which will be a problem. The winds will be strong overnight, about 25-30 knots (30mph)."
Emma Dobson, of Tavistock, Devon, called the conditions "horrific".
"Walking back to the actual campsite I got my wellies stuck and I had to physically pull my boots out of the mud," she said.
"It smells dodgy as well so I don't know what's been going into it.
"But it's all good fun. You can laugh about it."
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.