Updated

The governor sent in the National Guard to evacuate people from their sweltering homes Thursday after storms knocked out power to more than half a million St. Louis-area households and businesses in the middle of a deadly heat wave.

With forecasters expecting another day of 100-degree heat, utility crews raced to restore electricity, and Gov. Matt Blunt declared a state of emergency, granting the mayor's request to send in nearly 300 troops to take people to air-conditioned public buildings and to clear debris.

"We can't overemphasize the danger of this heat," Mayor Francis Slay said. "The longer the heat goes on and the power is out, the riskier it is."

The heat has contributed to at least 20 deaths across the country in the last week.

Police used public-address speakers from their squad cars to announce locations of the community centers and other places designated as cooling centers. Volunteers went door to door, checking on people with no power to run fans or air conditioners.

Click here to visit FOXNews.com's Natural Disaster Center.

Utility workers urged customers to find a cool place to stay. They warned that power could be out in some areas for three to five days.

The day's high was 97 degrees, but the humidity made it feel like 111. The region could get some relief on Friday, when the high was expected to drop to the mid-80s.

The storms tore through the city a day earlier, ripping off a section of airport roof and dumping it on a highway. Windows were blown out of a hotel restaurant and a press box at baseball's Busch Stadium. At least three buildings collapsed, and more than 30 people were injured.

"I've never seen this many people without power, this much debris, buildings collapsed, lines down," the mayor said.

By Friday evening, power had been restored to 160,000 customers, but new reports of outages kept coming in.

St. Louis-based AmerenUE, the utility serving Missouri and Illinois, said it would restore power to hospitals, nursing homes, water-treatment plants and other "critical facilities" first.

"If you're out of power, go to family, a friend or a cooling shelter," Vice President Richard Mark said. "Take whatever means necessary, but stay out of your home."

City Health Director William Kincaid cautioned that the city's older housing, much of it made of red brick, can heat up like furnaces in the summer heat.

"It could be a very dangerous day," Police Chief Joe Mokwa said.

John Swapshire, 39, grabbed the next-to-last window fan at a hardware store for $14.99. The electricity at his home was out, but he had a gas-powered generator.

"I had to go to six stores to get this. They were either closed because of the electricity or sold out," Swapshire said. "I don't think you can buy a cube of ice in all of St. Louis, either."

Stanley Shelton, 53, found a cool spot under a tree in a downtown park where piles of broken limbs and branches covered the grass.

"I've never experienced anything like this. I don't know anyone with power," Shelton said. "I'll just sit in my yard with a big jug of water and wait for it to pass. Maybe I'll take a couple cold showers. That works too."

The death toll from the heat wave that has gripped much of the country for the past week rose to at least 20 people in seven states. Four more people died in the Chicago area, bringing the total number there to seven, officials said. Two have died in the Philadelphia area, two in Oklahoma City, two in Arkansas, two in Indiana and one each in South Dakota and Tennessee.

In St. Louis, officials reported the death of a 93-year-old woman who had air conditioning but no power. In Indiana, a 25-year-old woman taking medications that might have affected her body's ability to stay cool died from heat exposure when temperatures inside her apartment reached 100 degrees, officials said Thursday.

Also Thursday, authorities in Kansas confirmed that two men in their 60s died from the heat. One was homeless and died in a Witchita park. The other did not have air conditioning.

In Wisconsin, a 6-year-old girl was killed Thursday when storms knocked part of a tree onto a tent at a park.

The storms also brought heavy rain, hail and 80 mph winds to Illinois on Wednesday night. Roughly 89,800 homes and businesses remained without power Thursday, but the weather was cooler in the central and northern parts of the state, with temperatures in the upper 70s or lower 80s.

Three people were injured in St. Louis when a residential building collapsed in a neighborhood south of downtown, police said. The historic Switzer building near the Mississippi River, once home to the famous licorice maker, also partially collapsed.

Many of the injuries were to baseball fans waiting for a St. Louis Cardinals-Atlanta Braves game. Winds blowing at nearly 80 mph blew out press box windows and ripped the tarp, injuring at least 30 people, five of whom were taken to hospitals, said Norm Corley, a supervisor with Accu-Care, which handles medical problems at the stadium.