Night comedy shows became the first casualties of the first strike by Hollywood writers against TV networks and movie studios in nearly two decades.
The walkout became inevitable late Sunday after last-minute negotiations between the Writers Guild of America and the Alliance of Motion Picture and Television Producers failed to produce a deal on such key issues as how much writers are paid when their shows are available on the Internet.
No new negotiations were scheduled, although the writers guild negotiating committee did plan a meeting of its members.
Nick Counter, chief negotiator for the producers union, said he expected a long standoff.
"We're hunkered down for a long one," he said. "From our standpoint, we made every good faith effort to negotiate a deal, and they went on strike. At some point, conversations will take place. But not now."
In Los Angeles, writers planned to picket 14 studio locations in four-hour shifts from 9 a.m. to 5 p.m. each day until a new deal is reached.
Writers said the next move was up to the studios.
"My hope is that it won't be too long," said John Bowman, chief negotiator for the writers. "We have more reason to get together than not."
Bowman said behind-the-scenes communication was occurring between the two sides with the hope of arranging more meetings.
Along with the shows hosted by Letterman and Leno, networks said other late-night shows bound for reruns included "The Daily Show," "Colbert Report," "Late Late Show with Craig Ferguson," "Late Night with Conan O'Brien," "Jimmy Kimmel Live!" and "Last Call with Carson Daly."
"Dancing With the Stars," one of the country's highest-rated prime-time shows, aired as planned on Monday night, with its hosts left to ad-lib.
The strike will not immediately impact production of movies or prime-time TV programs. Most studios have stockpiled dozens of movie scripts, and TV shows have enough scripts or completed shows in hand to last until early next year.
However, some producers were torn about trying to shoot those finished scripts.
Tim Kring, a producer and writer of the NBC hit "Heroes," said he had to revise the ending of the show's 11th episode on the chance that it might be the last one to air this season.
"Fortunately we were able to hustle back," Kring said from a picket line in an effort to shut down the show. "The audience won't be left in a lurch."
While scripted shows suffer from the strike, reality shows could flourish because they do not use union writers, despite an aggressive attempt by the writers guild to organize the staffers on the programs.
Viewers could also check out more entertainment on the Internet, ranging from user-generated fare on YouTube to professionally produced shows such as "Quarterlife."
Writers have not gone on strike since 1988, when the walkout lasted 22 weeks and cost the industry more than $500 million (EUR345 million).
The dispute has broad implications for the way Hollywood does business, since whatever deal is struck by writers will likely be used as a template for talks with actors and directors, whose contracts expire next June.
Talks began in July and continued after the writers contract expired last Wednesday.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969