Mexico City officials have shut down a main pipeline providing fresh water to millions of residents because reserves have fallen to record low levels.
The closure, due to last 36 hours, will affect five million people, or a quarter of the city's population.
Unusually low rainfall last year and major leakage are blamed for leaving reservoirs less than half full.
Hundreds of water trucks have been deployed in the areas worst affected by the cuts.
The local government says it will carry out emergency repairs to the water supply network.
More than 50% of the water carried by the pipeline leaks out before it reaches its destination.
This is the third time the capital has faced such a drastic form of water rationing this year, the BBC's Stephen Gibbs in Mexico City reports.
It has been deliberately timed to coincide with Easter weekend, when many residents, or at least those who can afford to, leave the city, our correspondent says.
Mexico City was once a floating city, built on a spectacular chain of volcanic lakes, and flooding used to be its main environmental threat.
But since the lakes were finally drained in the 1960s, the city has been struggling with its water supply, our correspondent says.
Are you in Mexico City? Have you been affected by the pipeline shut down? Send us your comments.
Story from BBC NEWS:
Published: 2009/04/10 02:27:23 GMT