People trying to get into Winchester's Boomtown Fair complained of sunstroke, dehydration, fainting, and a lack of toilets or water while stuck.
Organisers blamed the queues on increased searches and delays opening the site gates caused by bad weather.
But many people expressed their anger on social media and described the situation as "dangerous".
Charlie Hadley tweeted: "We have been queuing for THREE hours. My best friend just recovered from chemo and is flagging. What's going on????"
Richard Evans posted: "Boomtown festival is now a serious safety concern. People are fainting. My daughter has been queuing for nearly 6 hours."
Tommy Palmer said: "Boomtown queue currently expected to take 5 hours. Blinding sunlight with no water. This is just as dangerous as the security risk."
Acts headlining Boomtown this year include The Specials, M.IA. and Cypress Hill.
Organisers describe the event on its posters as "the most fully theatrical and immersive festival experience on Earth".
A festival spokeswoman said: "We were delayed in opening our gates early this afternoon due to the recent wet weather conditions.
"This delay in opening, along with the large-scale searching operation we are running this year have affected the time for people spent in the queues.
"The welfare of our attendees is absolute importance, we have been, and will continue to, send additional provisions and distributing water to those in the queues.
"Security, event management, medical and welfare teams are in position at all the gates and we are actively looking at ways to speed up the queues and get people into the festival."
Festivalgoers say they have had to queue for up to six hours to get into an event site.