Clear Channel Radio is using a two-day live music event in Las Vegas to kick off its relaunch of iHeartRadio as a competitor to Pandora and a formidable platform for advertisers.

Ryan Seacrest will host what Clear Channel is calling “the biggest live concert festival in radio history” at the MGM Grand on September 23 and 24, featuring an array of major name talent including Lady Gaga, Coldplay, Alicia Keys, Steven Tyler, Sting, the Black Eyed Peas, Kenny Chesney, John Mayer, Jennifer Lopez, Usher, Carrie Underwood, Jane’s Addiction, Nicki Minaj, David Guetta, Rascal Flatts, Bruno Mars and more.

Promotion of the iHeartRadio Music Festival kicked off Monday with a simultaneous announcement by all 850 Clear Channel Radio stations across the country, as well as on each of their websites and at iHeartRadio, starting a summer-long campaign. John Hogan, president and CEO of Clear Channel Radio, said this was the first time ever that the company had run a “roadblock” announcement like this.

Not incidentally, the announcement also demonstrated Clear Channel’s business clout and the advertising potential of a platform that can reach as a huge number of national and local consumers. Hogan estimated that 100 million people heard the initial spot.

The revamped iHeartRadio will deliver more than 750 broadcast radio and digital-only music, talk and comedy stations from 150 cities, along with the ability to create custom stations, in one integrated product. It will be available online, both at the existing iHeartRadio site and on all Clear Channel radio station sites, as well as on iPhone, iPod Touch, Android, Windows Phone 7, Blackberry, iPad and HP TouchPad mobile devices.

New iHeartRadio will debut by offering music live from the Las Vegas event. To help boost consumer adoption of the free service even further, the New iHeartRadio will be commercial-free through the end of the year.

Hogan said that New iHeartRadio’s ability to create custom stations will be better than any other custom radio station service because it included significantly more songs and offered more control than any existing services

“Music is the heart of our business and we think there is no better way to celebrate the launch of the new iHeartRadio than with a groundbreaking, multi-day festival featuring the world’s biggest, most exciting artists – and reaching the largest combined U.S. radio and online audience ever,” said Bob Pittman, Chairman of Media and Entertainment Platforms for Clear Channel Radio. “This underscores Clear Channel’s transformation to a dynamic integrated media and entertainment company with an unmatched reach in the U.S.”

From now until the September event, Clear Channel stations will be featuring exclusive artist and other interviews, holding ticket giveaways, and building anticipation in many other ways.

Clear Channel reports there are already more than 45 million listening hours a month from its 27 million unique website visitors and 30 million downloads of the current iHeartRadio mobile application, which streams about 750 music, comedy and talk radio stations.