The faithful lined up for the iTunes Festival at SXSW as early as 5 p.m. Tuesday, five hours before headliners Coldplay would appear on the ACL Live stage, not their first gig at the intimate Austin venue. A good thing, too, since security — as stringent as at some airport terminals — and the attendant video release process took quite some time. As such, the three levels of the theater, banked with audio and video equipment, were slow to fill for the first act — moody, insistent London Grammar — which fired up around 7:30 p.m.

Miranda Rieter and John Arsenault from Denver felt lucky to have gone through the lottery to obtain the wristbands and tickets, and they understood the reasons for all the extra gatekeeping. Robert Walker and Kelly Lux of North Austin, for their part, have sidestepped most of the SXSW madness. Coldplay, they felt, was worth the extra effort. Fernando Calvache and Rodrigo Jatene from Sao Paulo, Brazil were ecstatic to win the drawing and looked forward to the middle band, Imagine Dragons. Ben Krupa from Minneapolis’ Institute of Production and Recording is in town for SXSW Music was into all three bands. He is basking in the sunny weather after a particularly harsh winter up north.

At least for the opening acts, the audience remained remarkably well behaved, perhaps because so many of them had just come from five long days of SXSW Interactive or Film. Imagine Dragons did what they can to rouse crowd. If anybody could, this propulsive group could. And they did.

Front man Dan Reynolds thanked the assembled for enduring "fiasco" to get in, before ramping into "Never Changing Who I Am." Turns out, Imagine Dragons are even more immensely likable, musically infectious live. The balcony finally rose to its feet for and extended, syncopated "Radioactive"

Coldplay hit the stage around 10:30 p.m., wrapped in rapturous video. It was easy to forget that millions were watching this iTunes Festival on live stream. One wonders: What could they see? The vocal mix improved by the time Chris Martin sat down at the piano to lead off on "Paradise." "We’ve been away for a while," Martin said. "But I can’t imagine a better place to come back."

Martin was so loose tonight. So much more fluid than the last time he played ACL Live. The audience rose as one for an overwhelming version of "When I Ruled the World"

Martin to the live-stream audience: "I can see you across the world, man."