" NEW YORK (AP) — This year's iTunes Festival is ending with a roar, to be delivered by Katy Perry. The pop superstar, whose new single "Roar" is climbing the charts, will be the final act of the month-long London festival. It kicks off Sept. 1 with another diva, Lady Gaga, and will feature Robin Thicke, Elton John, Justin Timberlake, John Legend, Kendrick Lamar and the Kings of Leon, among others. Perry's "Roar" is her best-selling single on iTunes with more 544,000 copies sold domestically alone, according to Apple. Perry said she's pumped to be performing and noted that Icona Pop, who have the hit "I Love It," will be her opening act. Rapper Iggy Azalea will also appear. The annual festival will be streamed on iTunes festival app and available for download. ___ Online: http://www.itunesfestival.com "