PHILADELPHIA - Comcast Corp. customers can now watch several cable TV shows and movies over the Internet, a move aimed at helping the cable TV operator keep up with the flight of viewers to online video.

Comcast, the dominant cable TV provider in the Twin Cities, hopes that by making the service available starting Tuesday exclusively to subscribers, it can keep them from defecting to rival TV providers -- or to the Internet.

Comcast, which announced the service in July before reaching a deal for majority control of NBC Universal, becomes the first cable TV operator to offer cable content online at no additional charge.

Until now, programs available for free online have been generally limited to older movies or shows from over-the-air broadcasters.

Other subscription-TV operators with similar plans in the works include Time Warner Cable Inc. and Verizon Communications Inc.

The Comcast service, Fancast XFinity TV -- it was called On Demand Online when first announced -- will initially be available only to those who buy both Comcast's TV and Internet services.

Those customers will be able to access the programs on computers anywhere, even at the home of a friend who is getting broadband Internet service from another provider.

In six months, Comcast said that its cable TV customers who use another Internet provider also will have access.

Customers can authenticate up to three devices -- for now PCs, but mobile devices are possible next year -- to access the cable content.

All told, Comcast is making thousands of hours of programming available from 27 cable channels including HBO and Cinemax. But what's actually offered will vary by cable channel, with channels such as TBS offering shows the day after they appear on cable TV and others offering reruns from past seasons.

And what each customer can watch depends on the cable TV package subscribed.