The New Orleans Sewerage & Water Board says people in most of the city should boil tapwater before drinking it or doing anything else that might get it into their bodies.
The boil order covers the east bank, but not Algiers.
Spokesman Robert Jackson says it will last at least until Tuesday afternoon.
Officials say it's a precaution because the main treatment plant lost power for a few minutes Monday morning during maintenance work. Low pressure in water pipes makes it possible for contamination to get in.
The order means east bank residents should not drink, make ice from, brush teeth, bathe or shower, prepare or rinse food with tap water unless it has been kept at a rolling boil for a full minute.