Google’s (NASDAQ: GOOG) YouTube, the runaway leader in online video, has begun trials of a new live-streaming service, potentially laying the groundwork for a new heavyweight in the Webcasting space.

YouTube is alpha-testing its live-streaming service with four content partners this week, and product managers Joshua Siegel and Christopher Hamilton emphasized the preliminary nature of the trial.

“For the purpose of the trial, this offering will only be available today and tomorrow,” Siegel and Hamilton wrote in a blog post. “Based on the results of this initial test, we’ll evaluate rolling out the platform more broadly to our partners worldwide.”

The first trials of the service are set to begin at 11 am ET Monday, with a full complement of programming lined up from the alpha testers: Howcast, Next New Networks, Rocketboom and Young Hollywood.

The trials will invite users to append comments to the broadcast, which the content partners will monitor in real time. With that feature, “we can respond to your questions and comments instantly,” said Darlene Liebman, vice president of production at Howcast, an online repository of how-to videos.

Howcast is planning three live segments for the YouTube trial. One, geared for aspiring filmmakers, will feature behind-the-scenes tutorials on aspects of movie production. Another will feature a celebrity magician. The third will be a cooking segment with a New York chef.

For YouTube, the live-streaming feature could open a new channel for businesses, content providers and amateur users to broadcast live events. Should YouTube proceed with a more general release of the live-streaming service following the alpha trial, it would enter into direct competition with providers such as Ustream, Livestream and

YouTube has long been rumored to be preparing a live-streaming service, and has offered live broadcasts of select events in the past. Google has often streamed live footage of its product launches on YouTube, and the company has on occasion reached special arrangements to broadcast other events