Viacom is teaming up with T-Mobile for a massive year-end event streaming across Viacom's digital properties.

The eight-hour program, "The Big & Best of 2012: A Year End Celebration Presented by T-Mobile," will stream on MTV .com, VH1.com, ComedyCentral.com and CMT.com on Dec. 12, starting at noon.

Terms were not disclosed but a person familiar with the deal said it involved a multimillion-dollar investment from T-Mobile, making it one of the larger spends on a Viacom digital tent-pole event. Viacom has previously live-streamed events such as the O Music Awards, intended to honor the best digital music, and election night coverage from Comedy Central's "The Daily Show" and "The Colbert Report." The OMA's and recent election coverage each attracted more than 2 million viewers, according to Viacom.

T-Mobile will be using the platform to promote its unlimited data plans, which it says are especially relevant to Viacom's core millennial viewer. The ability to stream eight hours of video to mobile devices without incurring overage fees is an example of that , said Chris Ficarra, senior VP-integrated marketing at Viacom. "T-Mobile's 4G resonates with our millennial viewers who want to be connected all the time," he said.

Hosted by MTV News' Sway Calloway, VH1's Carrie Keagan and CMT's Kourtney Hansen, the event will broadcast live from MTV 's Time Square studio and music venues in New York City, Boston and Nashville. The eight hours will be filled with celebrity interviews, live music, year-end countdowns, contests and highlight packages from shows such as "The Colbert Report."

Carly Rae Jepsen, whose song "Call Me Maybe" was one of 2012's viral hit, will list her top five moments of the year. There will also be performances from rocker Grace Potter, country duo Thompson Square and indie-rock band Alt-J.

As the sole sponsor of the event, T-Mobile was heavily involved in its creation, said Kari Marshall, director of media and branded entertainment at T-Mobile. The company will run contests during "The Big & Best," giving away Galaxy IIIs every hour, and the hosts will highlight ways to use the phone to engage in social media and enjoy music and streaming video.

For Viacom, the program is another element in its strategy to provide "storytelling without borders." While the company wants to attract as many viewers as possible to the live stream, it's also measuring "The Big & Best of 2012," by how viral it goes. Mr. Ficcara said Viacom chose to stream the event digitally rather than putting it on its traditional TV properties because the content lends itself well to being clipped and shared.

All of Viacom's TV properties, however, will be directing viewers to the streaming event, which will be accessible both on the web and through mobile apps.

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CORRECTION: An earlier version of this article said Steven Colbert would host a live, online episode of "The Colbert Report" for the special. "Big & Best" will actually include three highlight packages from "Colbert."