LOS ANGELES, Nov. 2— Hollywood’s two decades of labor peace shattered Thursday night, as movie and television writers declared they would embark on an industrywide strike for the first time since 1988, when both writers and Teamsters walked out.

The writers’ union announced this afternoon that the strike would begin on Monday morning.

Unless there is a last-minute settlement, the strike will pit union writers, whose position has been eroded by reality television and galloping technological change, against studios and networks that are backed by big corporate owners like General Electric and News Corp., but are also unsure of the future.

The walk-out threatens an instant jolt to television talk shows like “Late Show With David Letterman,” which rely on guild writers to churn out monologues and skits. “The Daily Show With Jon Stewart,” “The Colbert Report,” “Late Night With Conan O’Brien,” and “The Tonight Show With Jay Leno” will all revert to repeats on Monday, at least for the time being.

And if the strike drags on, audiences could see the eventual shutdown of soap operas, TV series and movie productions, as they exhaust their bank of ready scripts.