Even as officials were assessing damage across the region, forecasters said another powerful storm packing high winds and the possibility of tornadoes was bearing down on portions of Mississippi, Alabama, north Georgia and eastern Tennessee.
Mayor Max Thompson said it appeared that a tornado touched down on the west side of Cullman and then cut through the heart of the city, which has about 15,000 residents.
"Downtown was hit pretty hard," said Freddie Day of the Cullman Police Department. He said a number of ambulances had been dispatched throughout the city. It was not immediately known how many people were injured.
Reports of people trapped in homes or overturned vehicles along with reports of downed power lines were coming in from every state in the region, according to emergency management officials.
Six people were killed in Alabama after storms moved through overnight and through the morning hours, according to the state Emergency Management Agency. One person was killed in Arkansas, officials in that state said.
Another five people were killed in storm-related incidents in Mississippi, according to the state Emergency Management Agency, which revised its death toll down from six earlier in the day.
"Our thoughts and prayers are with the families who lost loved ones or property in this devastating storm," said Mississippi Gov. Haley Barbour, who declared a state of emergency in 39 of the state's counties. The declaration allows the state to offer aid to the counties during recovery efforts.
The state was also bracing for flooding along the Mississippi River.
In Cullman, the severe weather caused a natural gas line to rupture, though the extent of the damage was not immediately known, said Day of the Cullman Police Department.
Officials at Cullman Regional Medical Center were assessing damage after declaring a "code d" -- a disaster -- at the facility, switchboard operator Sharon Barnett said. There were no immediate reports of storm-related injuries at the hospital.
One witness said the damage at the hospital was confined to the roof. "There's no actual building damage, just stuff that fell off the roof," said Summer Frost, who works at a Subway restaurant inside the hospital.
In Chattanooga, Tennessee, iReporter Erika Dunn said a tree fell on her great-aunt's home. She said her great-aunt reported the storm was so bad she could see a white wall of water coming toward the house. She was headed for her basement and was closing the door of her screened porch when the tree fell.
"It got really strong, really fast," Dunn said.
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While no one was hurt in that incident, others across the South were not so lucky.
There were reports of dozens wounded in storm-related accidents in Mississippi, Alabama, Arkansas and Tennessee.
In northeastern Alabama, there were reports of people trapped in homes in Marshall County after a possible twister touched down earlier in the day, said Lee Rosser, a logistics specialist for the county Emergency Management Agency.
Rescue crews were working to free a number of people trapped at Lake Guntersville State Park, where a number of RVs were parked at the time, said Yasamie August, a spokeswoman for the Alabama Emergency Management Agency.
The storm also damaged an airplane hangar at Birmingham-Shuttlesworth International Airport, airport spokeswoman Toni Bass said. Passenger operations were not affected, she said.
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Hundreds of thousands of people across the region were without power, including 269,000 in Birmingham, said Michael Sznajderman, spokesman for Alabama Power.
"We're chipping away" at restoring power, he said, but crews may be forced to halt work as a second line of storms approach.
CNN iReporter Paul Tinsley of Murfreesboro, Tennessee, said he and his wife were awakened overnight by "high winds that were picking up rapidly." He said they grabbed their dogs and ran to a downstairs bathroom. "I was convinced a tornado was seconds away from hitting the house," he said. "Luckily, that wasn't the case."
He said he left the house after the storm passed, and "there are an unbelievable amount of trees down in my neighborhood."
Meanwhile, the National Weather Service issued a tornado watch for portions of southeastern Arkansas, northeastern Louisiana and much of Mississippi until 7 p.m.
"The storms are just amazingly explosive and they're covering a very large area," said Greg Carbin with the Weather Service Storm Center in Norman, Oklahoma.