Usher kicked off the action around 1:30pm on Saturday (GMT).
The biggest draw of the day will be main stage headliner Madonna, as the queen of pop plays her only UK festival date of the summer.
30,000 people are expected over the two days, and more than half a million people applied for tickets.
The event is the biggest free ticketed music festival in Europe and has previously been held in Dundee and Preston.
Following a countdown by Radio 1 DJ Edith Bowman, the gates to Mote Park were opened at exactly midday.
Fans at the front of the queue sprinted to get to the front of the main stage.
Louise from Sittingbourne was first in the queue with her mates.
She told Newsbeat: 'We've been here since half past eight this morning and it is hot. We are looking forward to seeing Madonna and Scouting For Girls.'
14-year-old Chrissy from Gravesend came to the festival with her sister.
She said: 'It's the first time we've ever been to anything like this, we're waiting until the end of the show to see Madonna. The weather is beautiful. It's just a bonus really.'
Madonna's set list will include her current number one single 4 Minutes and some tracks from her chart topping album, Hard Candy.
Speaking to Radio 1 DJ Jo Whiley, she said: 'Obviously, I want to do new stuff because I’m very excited about it.'
'I feel that’s what people come to hear but I also want to do a few of my oldies but goodies.'
'I chose Hung Up because that was the biggest hit from my last record and I chose Music because it’s a crowd pleaser, it’s anthemic and it brings people together.'
She's staying tight lipped on whether she'll sneak her current collaborator, Justin Timberlake, into the show.
From BBC Newsbeat