The campsite gates opened on Thursday night, with 40,000 turning up early to grab the best spot for the festival, now its 21st year, which will include performances by Biffy Clyro, Pixies and Ellie Goulding.
Other acts appearing over the weekend include the headliners of T in the Park's first year at Balado - The Charlatans and Paul Weller - with both acts taking to the stage 18 years to the day after they performed at the festival's debut in Kinross.
Nine people were arrested and 137 sought medical attention as the first full day of the festival got under way.
Nick Moore, of the Scottish Ambulance Service, said the high temperatures - with the mercury reaching 22°C - had meant that some people needed treatment for sunburn, while others had suffered minor cuts and grazes.
Police Scotland Chief Superintendent Kevin Lynch, the Event Day Commander, said: "Everything has gone very well so far and the crowds are in really good spirits. We're doing everything to ensure people are safe and would also ask visitors to be aware of their own personal safety."
"Crime numbers are level with the same time last year and there have been nine arrests, mainly for minor offences."
Organisers said the event will act as a "huge send-off party" before a move to the Strathallan Castle estate in Perthshire in 2015.
More than 2,800 artists have played Balado, including Foo Fighters, The Stone Roses, Rihanna, The Killers, The Who, Kylie Minogue, Oasis and Eminem.
Biffy Clyro have played the most, with their set last night taking their total to 10.