Liam Gallagher's band Beady Eye were the surprise opening act on The Other Stage with a raucous set at 11:00 BST.
"It's never too early for a bit of rock'n'roll aggro, is it?" asked Gallagher, as his band launched into Oasis hit Morning Glory.
Other acts playing on Friday include Chic and headliners The Arctic Monkeys.
After a day of rain, the clouds broke around midday, apparently conjured up by the sun-bleached rock of US sibling trio Haim. By the end of their set, raincoats were being traded for suncream.
The sun came out in time for fans to watch Jake Bugg's afternoon set
About 180,000 fans have gathered at the 900-acre Worthy farm for the festival, which has been largely peaceful so far, with 107 reported crimes and 61 arrests.
Somerset police say there have also been 32 drug related offences reported, 12 of which were for possession of restricted substances.
Liam Gallagher's early morning set was something of a turnaround for the former Oasis frontman, who previously headlined the festival in 1995 and 2004.
The latter appearance was not well-received, and Gallagher later claimed the festival was "full of idiots".
But after Friday's performance, the singer told the BBC "Glastonbury's back in the good books. I just had a bad experience, but now it's cool again.
"It's always been cool, it's just me. I'm full of it."
He added that playing such an early starting time on the Friday morning had been something of a surprise.
"Someone just asked us - do you fancy going on at 11 o'clock, and we thought, why not? We've headlined it, we've done all that. It's time to do something different.
"I absolutely loved it, I mean it. I thought 11 o'clock could be really ropey but I thought we done well, man."
Click here to watch a short video of the band talking about the festival.