The ads, which appear 15 seconds after a user begins watching a video clip, take the form of an overlay on the bottom fifth of the screen, not unlike the tickers that display headlines during television news programs.

A user can ignore the overlay, which will disappear after about 10 seconds, or close it. But if the user clicks on it, the video they were watching will stop and a video ad will begin playing. Once the ad is over, or if a user clicks on a box to close it, the original video will resume playing from the point where it was stopped.

“What we have come up with is a user-controlled ad format that is engaging,” said Eileen Naughton, Google’s director for media platforms. “We want our users to be able to accept and choose what type of advertising they engage in.”

For now, Google will place the ads only on video clips of its content partners — the more than 1,000 small and large media companies that have licensed their videos to YouTube. By doing so, YouTube will avoid the potential liability of having ads appear on copyrighted clips it is not authorized to display. And it will also prevent ads from playing on clips generated by users whose message may not be to the liking of advertisers.

Image A demonstration of an ad for a movie on the bottom of a YouTube video. Real ads would not reflect the content of a video.

The revenue from the ads will be split between the media partner and YouTube. Ms. Naughton said Google would charge advertisers $20 for every 1,000 times the ads were displayed. Google said the ads would begin appearing today throughout the site. Ms. Naughton also said advertisers would be able to take aim at specific channels and genres, as well as demographic profiles, geography and hour of the day.