They were greeted by blistering sunshine, which was forecast to continue today when the festival officially gets under way, with acts including Biffy Clyro, Ellie Goulding and Pixies to perform.
The gates opened an hour and a half early, at 1.30pm, to a queue of around 15,000 people. By the evening, 40,000 campers had arrived.
T in the Park will move to Strathallan Castle, near Auchterarder, after 18 years in Balado, from next year.
A spokeswoman for the festival said: "We opened early due to the size of the queue, we wanted to get people in there as quickly and safely as possible. The camp site opening is one of those moments everyone looks forward to and there was a big cheer when the gates opened up. It was a lovely atmosphere and everyone was looking forward to seeing some great acts. It was also a bit emotional as it was the last camp site opening we'll have in Balado, so all these little milestones mean all the more. But everyone was having a great time and spirits are high."
Revellers at the 85,000 capacity event may need their wellies tomorrow, when Calvin Harris and Paolo Nutini headline, with heavy rain forecast in the evening.
Weather is expected to be largely overcast on Sunday, with sunny intervals in the afternoon, ahead of the closing performance from Arctic Monkeys.
Organisers yesterday said that they had released an extra batch of day tickets for tomorrow and Sunday due to "phenomenal demand".
Meanwhile, Police Scotland launched a new campaign, urging festival-goers to stay safe at summer music festivals. The force has warned against the use of drugs and legal highs, carrying weapons or using smoke canisters or flares at festivals.
It also said care should be taken with personal belongings, music fans should stay close to their friends and that ticket touts should be avoided.
Superintendent Danny Hatfield said: "By taking a practical and pragmatic approach to personal safety and being aware of the potential risks, I am confident festival-goers will fully enjoy these events.
"Police Scotland and event staff will be working hard again this year to make sure festival goers have a summer to remember for all the right reasons. They can play their part by looking after each other, drinking responsibly, and not taking drugs. We want people to enjoy themselves safely by following our advice."