By Steve Gorman

LOS ANGELES (Reuters) - Striking Hollywood writers forced TV studios to halt work on at least seven prime-time sitcoms by Wednesday as the effects of their three-day-old walkout grew more swiftly than many in the industry had predicted.

There was no sign of back-channel talks between studio chiefs and union leaders being renewed, stoking fears that Hollywood's first major labor strike in 20 years could be a prolonged one.

As production ceased on such TV shows as the CBS hit "Two and a Half Men," the Fox network's new Kelsey Grammer comedy, "Back to You," and NBC's popular workplace satire "The Office," hundreds of crew members and cast from those series began receiving layoff notices.

"We shut it down yesterday," said Greg Daniels of "The Office" as he walked a picket line in Burbank with more than 100 other show runners -- the writer-producers who oversee day-to-day production of TV programs.

He said NBC had two more original "Office" episodes left to broadcast "and then we're done."

Other early sitcom casualties, hit hardest because they depend on a substantial last-minute script rewrites, included "'Til Death" on Fox and CBS entries "The New Adventures of Old Christine," "Rules of Engagement" and "The Big Bang Theory."

Some have enough new episodes produced in advance to last a few more weeks without repeats, leaving viewers largely unaware of the strike fallout for the time being.

But the camera operators, hairstylists, grips, production assistants, electricians and other crew members on those shows now join the ranks of 12,000 screenwriters out of work since their union, the Writers Guild of America, went on strike against major U.S. film and television studios on Monday.

The WGA's three-year contract expired last Thursday. Negotiations collapsed on Sunday night as the two sides' failed to reach a deal on writers' demands for a greater share of revenues from the Internet, widely seen as the future distribution channel for most entertainment.

Nightly talk shows that rely on a steady stream of topical jokes and skits were the first to be affected. At least eight such programs, including NBC's "The Tonight Show with Jay Leno," were forced into reruns the first day of the strike.

Dozens of scripted weekly series -- comedies and dramas representing the bulk of major networks' prime-time schedules -- will be essentially be forced out of production "in the next three to four weeks," said Marc Cherry, creator and executive producer of the ABC hit "Desperate Housewives."

"We finish tomorrow," Cherry, a WGA negotiating committee member, said of his show. "It breaks my heart. I love my cast. I love my crew. We're having a really good season ... and it's kind of inconvenient and annoying all the way around."

TV networks have said they had enough original episodes stockpiled to keep many shows on the air without reruns until December, or even January and February.

But many show runners, who are union members, are refusing to return to work even to provide non-writing services needed to put the finishing touches on incomplete episodes.

"Desperate Housewives" has enough completed episodes to run through the first week of December, Cherry said. "Viewers won't notice until the second week of December, and suddenly we won't be there anymore."

NBC had insisted that "The Office" remained in production during the first two days of the strike even though its star, Steve Carell, had joined writers on the picket line.

"They spent hundreds of thousands of dollars yesterday and the day before keeping the show open and filming about 30 seconds of material, just so they wouldn't have to say the show was shut down," said Daniels, the show runner.

(Editing by Alan Elsner and Chris)