The spigot will run dry, toilets won’t flush and there will be no cooling shower on sweltering days for more than 100,000 people in Prince George’s County as crews wrestle to repair a major water main that serves their homes and businesses.

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.

The problem occurred in a week when Washington’s temperatures were expected to soar into the 90s, with heat indexes topping 100 degrees.

Andrews Air Force Base and National Harbor are within the affected area.

Scott Peterson, spokesman for Prince George’s County Executive Rushern L. Baker III (D), said the “economic impact of this event will be the equivalent of a natural disaster hitting the county.”

County officials said they will open at least two “reception centers” and deploy tanker trucks to provide water.

“We know residents and businesses are frustrated,” Baker said. “We are frustrated, too.”

But Baker said this was not the time to assess blame and that the priority was helping residents through the crisis.

According to a map provided by WSSC, the water main supplies much of the southern and western parts of Prince George’s between Andrews and the Potomac River and from the border with the District to several miles south of the Capital Beltway. In addition to serving Andrews and National Harbor, the pipe carries water to the communities of Morningside, Hillcrest Heights, Camp Springs, Forest Heights, Temple Hills and Oxon Hill.

“No question, this is going to be rough on people for several days,” said I.J. Hudson, a WSSC spokesman.

The massive convention hotel, restaurant and shopping complex at National Harbor is preparing to curtail service, Senior Vice President Kent Digby said. He said the outage will cripple businesses in the 300-acre complex that need clean dishes and clean linens.

He said the center’s hotels are planning to shut down and are redirecting guests who are scheduled to stay at the hotels in the coming days.

The more than two dozen restaurants will remain closed Wednesday for the duration of the water shut-off, Digby said.

“We’ll just have to ride this thing out,” he said.

One of health officials’ major recommendations for coping with the heat is ample water consumption — remaining hydrated.

Hudson of the WSSC said that people in the main’s service area should stockpile at least two gallons of water per person for every day they will be without service. The drinking water remains safe, Hudson said.

Jamie Miller, a spokesman for Giant Food, said the company began working with bottled-water suppliers Monday night to step up deliveries to stores in the area.

Prince George’s fire officials said any residential or commercial fire draws at least seven fire trucks that carry a total of 2,000 gallons of water. That is sufficient for most fires, the department said, but as a precaution, tanker trucks capable of holding 3,000 to 7,000 gallons of water will be deployed as well. Firefighters also will be ready to draw water from nearby streams or swimming pools.

While the WSSC planned to have the pipe shut off Tuesday night, Hudson said, the taps would not go dry immediately. The system has about 12 to 14 hours’ worth of water, he said. People are being asked to restrict their water use during that time to stretch out the supply.

WSSC crews stood ready at valves throughout the day in case they needed to shut off the pipe in an emergency, Hudson said.

The pipe is in a wooded area just inside the beltway, between Forestville Road and Suitland Parkway. It is “not that far” from the Beltway, Hudson said, but it is downhill from the road, so any water from a break would flow away from it. No buildings are in the immediate area, he said.

Because such pipes are so large and carry so much water under pressure, they literally can explode. WSSC engineers, who have sought building restrictions within 80 feet of such pipes, have compared the force of such a blast to a bomb that can hurl rocks and other debris “like shrapnel.”

The main, which was installed in 1965, is unusual in the WSSC system because there is no way to route supplies around it, he said.

The utility was first alerted to a problem with the pipe on Thursday, when acoustic cable that had been installed just a few weeks earlier “started to pick up a bit” with alerts, Hudson said. The equipment is designed to detect the sounds of the concrete pipe’s reinforcing steel wire as it begins to snap from corrosion, signaling that the pipe is weakening in that spot.

As of Monday night, the pipe had registered 30 alerts “in fairly quick order,” Hudson said.

“It tells us this pipe is headed out,” he said. “We can’t just sit back and wait.”

WSSC spokesman Jim Neustadt said the prestressed concrete cylinder pipe involved was essentially the only water source for the area served, making it necessary for people “to fill up on water” ahead of the shutdown.

Before the flow of water through the pipe is cut, WSSC would try to fill up its storage facilities so that some water could be delivered through smaller pipes, said Jerry Irvine, another utility spokesman.

This is at least the third time in the past several years that acoustic equipment installed in a pipe has alerted the WSSC to a potential break, allowing for a preemptive shutdown.

One such shutdown occurred in the Rockville area this spring, utility officials have said. In 2010, water restrictions were imposed over the Fourth of July weekend after an eight-foot-wide pipe was shut down in Potomac.

The utility is still reviewing the March break of a five-foot-wide pipe in Chevy Chase, where the force of the blast blew a crater 20 feet deep in a side street off Connecticut Avenue and created a 40-foot geyser. In that case, utility officials said, no warning was received because the break occurred near a joint where there was no wire being monitored.

The main that triggered the latest alarms is inside the Beltway between Suitland Parkway and Forestville Road. The WSSC said it is in a relatively inaccessible spot, and officials said a road was constructed by Tuesday morning to bring in repair equipment.

The WSSC’s 350 miles of prestressed concrete cylinder pipe form the backbone of its distribution system for 1.8 million people in Montgomery and Prince George’s counties.

Lori Aratani, William Branigin, J.D. Harrison, Michael Laris, Trishula Patel, Miranda S. Spivack, Martin Weil and Ovetta Wiggins contributed to this report.