The Washington region suffered through another day of stifling heat Sunday, and thousands of people went without air conditioning as utilities called in reinforcements and the National Guard pitched in to help with storm cleanup.

The heat index topped 100 again, and sultry weather was forecast for the entire Fourth of July week as legions sought refuge from sweltering homes and waited for work crews to make repairs.

In Washington, the city canceled public school summer classes and programs for Monday, citing power outages. Twenty members of the National Guard were sworn in to patrol intersections with broken traffic signals.

By late evening Sunday, about 538,000 Washington area households and businesses — about one in four of the region’s electric customers — were still without power, according to data on utility Web sites.

Montgomery County also canceled summer school classes and programs for Monday and Tuesday, and Prince George’s County did the same for Monday.

The federal government said its offices in the area would be open, but employees have the option to take unscheduled leave or do unscheduled telework.

The loss of hundreds of traffic signals and some 911 service, as well as numerous road closures, could make for a chaotic commute Monday morning, officials said.

Many households had power restored Sunday, but the area remained partially crippled by the vast crescent of storms that swept across the Appalachians on Friday night, killing at least five people in the region. And Sunday, as people perspired, dogs panted and officials said it might be a week before all power is restored, politicians vowed to try to speed things along.

Many people used generators to produce electricity. Five people in the District were taken to hospitals Sunday with carbon-monoxide poisoning, and officials said generators should never be operated indoors.

Sunday was the area’s fourth day of 90-degree-plus readings, with more such weather in sight and a continuing chance for more storms. Just before 4 p.m., the temperature at Dulles International Airport reached a record for the day of 96 degrees, breaking the record of 94 set in 1968.

The National Weather Service predicted slightly cooler readings for Monday. But it warned of extremely hot and threatening weather again midweek, with heat indexes of 105 and readings in the 90s through the weekend.

Meanwhile, utilities scrambled to borrow extra repair crews from nearby states, but fewer were available because of the extent of the storm’s damage.

The highest level of outages at one point Sunday — 60 percent — was reported for Pepco’s service to Montgomery.

As of 10:30 p.m. Sunday, a total of 167,668 customers with Pepco, Baltimore Gas and Electric and Potomac Edison in Montgomery remained without power.

Across Prince George’s at that time, 103,361 Pepco and BGE customers customers were still without power.

There were 208,285 Dominion Virginia Power and Northern Virginia Electric Cooperative customers without power at 10:30 p.m. And in the District, Pepco reported 58,977 without power.

Pepco said it requested 1,000 additional crew members but initially found fewer than 200 available. Many of the utilities Pepco typically relies on for “mutual aid” in Ohio, Pennsylvania and New Jersey were also severely affected by the storm.

One caravan of repair trucks from Connecticut got stuck in a giant traffic jam Sunday on the New Jersey Turnpike and faced a substantial delay in arrival.

“What we’re asking for, everyone is asking for, because we’re all in the same position,” Thomas H. Graham, Pepco region president, said at a news briefing Sunday. He added that crews were en route from as far away as Missouri, Oklahoma and Canada.

Saying the damage to Pepco’s system had been “extraordinary,” he estimated that 90 percent of affected customers would have their power back by Friday night.

But Montgomery County Executive Isiah Leggett called that “unacceptable.”

Flanked by Maryland Gov. Martin O’Malley (D), Leggett said at a news briefing in Gaithersburg that almost two days after the storm, more reinforcement crews should be in place. “We have known since 40 hours ago that the storm hit and we didn’t have the availability of crews on the immediate area,” Leggett (D) said. A week-long power outage in such scorching weather was “an unacceptable number. . . . That’s setting the bar too low.”

“I will not accept the timetable of July the sixth,” he said. “Today is just July the first. Having our citizens go seven days without utilities, in my opinion, is not the kind of service we should expect.”

O’Malley said the problem was that the storm produced effects akin to those of a hurricane, but it descended on the area without an early alert. “This sudden storm had a hurricane impact without a hurricane warning,” he said. “Nobody predicted this storm would be of the magnitude it was or the violence it was.”

Graham said: “You can’t say this was a Pepco issue. It’s a catastrophic weather event that millions of individuals are experiencing right now. . . . I can only think of one or two occasions when the damage has been greater.” He cited Hurricane Isabel in September 2003 and a January 1999 ice storm.

In Virginia, Rodney Blevins, Dominion Power’s vice president of electric distribution operations, said the storm caused the third-worst power failure in company history. It is the only event in its five largest mass outages not caused by a hurricane, he said. Blevins said Dominion has secured 1,000 additional line workers, coming from 13 states and Quebec. He estimated that 80 to 85 percent of Dominion customers still in the dark will have power restored by Tuesday night, with nearly all restoration done by Saturday. “A lot of hard work happens in that last 5 to 10 percent,” he said.

Across the area, people whose homes lacked power snapped up free bags of ice, moved to air-conditioned hotel rooms and drove around in air-conditioned cars to escape the heat.

“I’ve been living in the swimming pool,” said Bob Ambrosini, who had no power in his Great Falls home.

Fairfax officials said Sunday morning that 911 service, which was disrupted as a result of the storm, was only partially restored. People who could not reach 911 were urged to call 703-691-7561 or 703-691-3680.

The failure of 911 service in Northern Virginia on Saturday cut off many residents in Fairfax, Arlington and Prince William counties from emergency operators from about 6 a.m. until partial restoration started in early afternoon.

“I don’t ever remember the 911 system going down, and it happened exactly at the time when we needed it most,” said Sharon Bulova (D), chairman of the Fairfax County Board of Supervisors. “Why was there not a backup or something? That’s a question regional leaders will be focusing on with our state partners in the aftermath of the storm.”

Steve Souder, director of Fairfax County’s 911 Center, said that the county was prepared to respond to emergencies but that it depends upon the Verizon network to convey the calls. “The storm itself, we handled quite well. We rocked and rolled and handled a zillion calls,” he said. When the 911 service went down about 6 a.m. and non-emergency phone lines also failed, “it literally made us dead in the water.”

A Verizon spokesman said that a power failure in one of its Arlington facilities caused technical and mechanical damage that resulted in the 911 outage but that the company has been working around the clock to restore full service.

The Fairfax and Prince William 911 call centers are receiving most emergency calls, said Harry J. Mitchell, Verizon’s director of public relations. But Manassas and Manassas Park are still without 911. The facility that went down “provided routing for the 911 call centers. Some 911 calls were sent without addresses,” Mitchell said. “Full power is now back on, and we’re working to resolve whatever issues remain.”

The 911 failure was a unique event, he said. “We have extensive plans for backup power, and they work without a hitch most of the time. In the case of Arlington, this issue affected both our primary and backup systems.”

Although plenty of gas stations reopened Sunday, one in Annandale stood out: It was charging almost $4 for a gallon of regular unleaded, at least 50 cents more per gallon than the other stations.

“There’s a shortage of gas in the area,” said the manager of the Annandale Exxon station on Little River Turnpike, just inside the Beltway. He declined to give his name or comment further.

“It’s absolutely ridiculous,” said Mike Nolan of Annandale. It took him $95 to fill up his sport-utility vehicle.

Still, there was no shortage of customers along an otherwise electricity-free stretch of Annandale sometimes called Korea Town. Nearby stations on Little River Turnpike were dark.

“It’s a rip-off,” said Abdi Rashid of Annandale. “I don’t know why they’re doing that.”

He had two children in the car, ages 15 and 1¹ / ₂. His home had no electricity. “We have to do it,” he said about paying the high price. “You have to go outside to get food for the kids. To go outside, you have to have gas.”

He said the same station was charging $3.49 on Friday.

Along a long stretch of Little River Turnpike in Fairfax and Annandale, and Columbia Pike in Annandale and Falls Church, most stoplights remained dark except at major intersections. Strip malls and fast-food restaurants that would normally be bustling on a Sunday afternoon were deserted.

But drivers were starting to get into the habit of stopping at intersections, allowing cars from side streets to enter without incident. Doug Henken of the Seven Corners area said he drove west to Centreville in search of a cool place to eat Saturday night.

All along Route 50, he said, “people were behaving themselves. I was shocked.”

Emma Brown, Mike DeBonis, Marc Fisher, Hamil R. Harris, Tom Jackman, Anita Kumar, Donna St. George, Patricia Sullivan, Ted Trautman, Martin Weil, Ovetta Wiggins and Mihir Zaveri contributed to this report.