The shutdown was carried out Tuesday night and may last up to five days, water officials said.
The warning triggered a headlong rush Tuesday to stock up on bottled water, fill bathtubs and gather any container that might hold water. Firefighters, hospitals and other services that depend on the water supply made contingency plans to handle emergencies.
The Washington Suburban Sanitary Commission, in a warning first issued Monday night, said a 4 1/2-foot main just inside the Capital Beltway was about to fail and might take as long as four days to replace. The utility said it planned to fill other pipes in the area that would be affected by the shutdown to capacity before cutting off the water late Tuesday.