Tens of thousands of people in the nation’s capital were warned against drinking their water Friday, prompting anger and confusion among D.C. residents who said they weren’t notified promptly of a potential contamination of the city’s water supply.

D.C. Water continued to advise those in a wide swath of Northwest and Northeast Washington to boil their water until further notice. John Lisle, a spokesman for the agency, said the warning would probably not be lifted until Saturday at the earliest.

Lisle said there have been no reports of sickened customers or other evidence that a temporary drop in water pressure in parts of the system had put District residents at risk. But he said the advisory to boil tap water would remain in effect until tests ruled out that bacteria or other contaminants were in the water supply.

Despite the absence of immediate danger signs, some D.C. residents and public officials said they were worried about how the utility and District government had handled the notification process — with a drawn-out trickle of warnings, some issued in the middle of the night on Twitter, that did not directly reach some affected people until more than 12 hours after the problem was detected.

The government’s competence in alerting residents of emergencies is especially important to residents of the District, a densely populated capital and potential target for terrorism.

“We’re concerned, and I think a lot of people are actually furious, that there was not a real effort to notify thousands of people who were drinking the water when they got up in the morning, making coffee and giving their kids the water,” said Erik Olson of the Natural Resources Defense Council.

Even mildly contaminated water that doesn’t harm healthy adults could be dangerous for others, such as infants, senior citizens and people with compromised immune systems, Olson said. “Clearly, there needs to be a better game plan to get the word out when there is a boil-water alert,” he said.

Sasha Lezhnev, a 38-year-old Columbia Heights resident, said he got a call from D.C. Water at 8:30 a.m., about 12 hours after the malfunction and more than four hours after the utility put out a boil-water advisory on Twitter.

Lezhnev said he and his wife give filtered tap water to their 9-month-old son, Leo, and could have given the baby contaminated water had they not already made baby formula.

“It’s irresponsible that they only called me 12 hours after the problem occurred,” he said.

Utility officials said they discovered at about 8:30 p.m. Thursday that a broken valve at the Bryant Street Pumping Station south of McMillan Reservoir had caused pressure in some pipes to drop. Low pressure creates a risk that contaminated groundwater can seep into the pipes, Lisle said.

The valve was working properly again about an hour later. At 10:59 p.m., D.C. Water tweeted that a “temporary problem” at the pumping station had caused a drop in water pressure and stated, “More updates to follow.”

Five hours later, as most of the city slept, the utility updated its website, sent emails to journalists and put out a tweet with a warning to boil water. Shortly after 4:30 a.m., the D.C. Homeland Security and Emergency Management Agency issued emails and text messages to 104,000 people who had signed up to receive alerts.

By 6:40 a.m., some people were having problems accessing D.C. Water’s website, prompting the utility to advise customers to use a specific type of browser to view the site. “Our website seems to be loading on Chrome, but not Safari,” the utility tweeted.

Lisle said robo-calls to customers began at about 6 a.m. Friday, but that the utility’s system can only make the calls over an extended period. Some customers reported not receiving calls until close to noon on Friday.

“I don’t know the exact number of hours, but it would take a long time,” Lisle said of the robo-call technology. “It’s not a perfect system.”

D.C. Council member Mary M. Cheh (D-Ward 3), who chairs the council’s Committee on Transportation and the Environment, sent a letter to D.C. Water General Manager David Gadis on Friday that said the episode had caused “extreme anxiety” among her constituents and that many said they learned of the warning to boil water only indirectly, and after they or their children had consumed tap water.

“It seems the public was not informed in a timely manner,” Cheh wrote, adding a list of specific questions about the incident and the notification process.

The office of D.C. Mayor Muriel E. Bowser (D) defended the city’s handling of the water problem in a written statement. “As soon as D.C. Water made the determination that, as a precaution, they would issue a boil water alert for part of D.C., we began spreading the word through our own alert systems,” the mayor’s office said. By early Friday morning, “news about the boil alert was on the television, radio, online news and on social media,” the statement said.

At a news conference Friday morning, Gadis said it had taken time for officials at the utility to determine who might be at risk and that they did not want to stir widespread panic.

“We didn’t want to send an alarm to people who weren’t affected,” he said.

D.C. Water initially said the warning applied to those who live within a large area across the top of the city, from midtown in the south to Military Road in the north and from Potomac Heights and Georgetown in the west to the eastern boundary of the District.

That shifted by midday, when the utility issued a revised and interactive map, narrowing the affected areas to a band stretching from the Potomac shore, through parts of Georgetown and into Adams Morgan and Columbia Heights and then east through Edgewood and Brookland.

The effects of the warning were widely felt at a time of year when water is in high demand across a sweltering city. D.C. officials said they closed 14 pools and 16 spray parks in the impacted areas. Warnings were put up at libraries not to drink from fountains. Water bottles were sent to summer schools and camp sites.

Several major hospitals were in the areas covered by the alert. MedStar Washington Hospital Center spokeswoman So Young Pak said the hospital “either experienced low water pressure or lost water for about an hour to the building where Labor and Delivery is located.” She added, “We made bottled water available to our patients and staff.”

MedStar Georgetown was delivering water bottles to its patients as employees took “extra steps before and after hand washing to ensure sterile hand hygiene,” Marianne Worley, a spokeswoman for the hospital, said in an email. The hospital also stopped serving coffee, tea and any other foods prepared or processed with water.

Amy Goodwin, a spokeswoman for Children’s National Medical Center, said it closed ice machines and water stations and instructed patients and staff to use bottled water.

Store shelves in supermarkets and drugstores in the affected areas were quickly emptied of water in various forms — flat, carbonated and flavored.

Some bars and restaurants, meanwhile, appeared to take the disruption in stride.

“We’re scrambling, but we’re opening,” said John Andrade, who owns four restaurants in the boil-water zone. He said workers at some of his restaurants — which include Meridian Pint, Brookland Pint, Rosario and Smoke & Barrel — had started boiling water for cooking and washing to get ready for the dinner crowds.

A sign on the front door of Tryst, a cafe on 18th Street NW in Adams Morgan, warned patrons the restaurant couldn’t serve tea or coffee.

“But hey,” it went on, “we still have booze!”

Michael E. Ruane, Teo Armus, Fritz Hahn and Fenit Nirappil contributed to this report.