Mainstream radio broadcasting is aggressively staking its turf on the frontiers of Internet broadcasting.

This week, Clear Channel Radio launched a new music site -- erockster (erockster.com). Aimed at 13- to 34-year-olds, it's an eclectic mix of classic and indie rock, soul, dance and hip-hop: A sampling of artists includes Radiohead, Arcade Fire, Coldplay, Fatboy Slim, the Ramones, Pink Floyd, Bjork, Portishead, Prince, Beastie Boys and The Kinks.

With its combination of mainstream, indie and edgy music, it's designed to draw a wide swath of people -- both those who listen to traditional radio formats, and those who have abandoned them for the more diverse options online.

In addition to the streaming player, listeners also can use an on-demand feature and choose individual songs or entire CDs.

The erockster playlist currently weighs in at more than 800 tracks, and will eventually feature a library of more than 5,000 songs.

The creative programming forces behind erockster are actor Eric Szmanda of "CSI: Crime Scene Investigation" fame, and partner Ty Malin.

The site features a list of the last 10 songs played and a complete playlist of what's available. Through the playlist, users can also link directly to iTunes and ringtone downloads, along with music videos, for many of the songs.

Starting today, erockster will stream from Clear Channel radio station sites in Los Angeles, Washington and Philadelphia, with other markets to follow.

It also will have an on-air presence, with plans for a weekly two-hour syndicated radio program in the works. And it will broadcast as one of Clear Channel's many HD multicast channels. HD channels are secondary channels that enable broadcasters to broadcast additional formats on the same frequency. These secondary channels require an HD receiver to tune them in.

Erockster has an interactive social networking component, where users can rate and comment on music.

Another radio giant -- CBS Radio -- also is establishing an online foothold on several fronts.

This summer, CBS will launch play.it, a music streaming service where users can indicate who their favorite artists are and get a customized playlist of similar music they may like.

Play.it users will be able to choose from a selection of icons representing different artists. By dragging the icons to their playlist, they'll hear a lot of songs by that artist, as well as other music influenced by or similar to it.

It's similar to online music site Pandora, which has been successful at matching its users' tastes in music with new content.

In March, CBS Radio and AOL announced a partnership in which CBS will power AOL's online radio network, replacing XM Satellite Radio. And CBS is planning to stream its HD channels, making that programming accessible to online users.