At least two tornadoes ripped across northern Texas Tuesday, flipping huge trucks like toys, tearing off roofs and grounding flights but apparently causing only minor casualties.

One twister touched down in a parking lot full of tractor-trailers -- large trucks that move shipping-container sized cargos -- and television images showed several large vehicles twisting and flying dozens of feet up into the air.

"It was raining and hailing like I said... and then all of a sudden, the rain and hail ceased... that's when we saw the tornadoes touch down," witness Jonathan Cook told CNN.

The resident of Burleson said he dashed out of the bank when he saw hail the size of nickel coins start falling and leapt back into his truck to get it to the cover of a nearby gas station.

"We were just under the gas pump awning, hoping it didn't come our way. We had a good visual of it, but we really had nowhere to go... we just had to run in the gas station."

Texas residents appeared mostly to have heeded warnings as the National Weather Service sent out an alert saying "Tornado Emergency. Two tornadoes are currently affecting the DFW (Dallas-Fort Worth) Metroplex. Take cover now!"

A tornado watch was issued for the Dallas-Fort Worth area -- home to more than six million people -- until 8:00 pm (0100 GMT Wednesday).

On his drive home Cook said "there were large trees, large tree limbs, a lot of leaves scattered across the highway, and tree limbs on the highway," as well as "many roofs taken off homes."

The area utility company, Oncor, reported more than 48,000 people were without power in the metro area. Dallas police warned residents to stay away from dangling power cables.

The tornadoes caused major damage in several metropolitan areas, including Kennedale, Arlington and Lancaster, local media reported.

But despite the powerful storm, casualties appeared to be minimal.

"At this point, there are no major injuries reported," Arlington Mayor Robert Cluck said in a statement.

"However, it is still early" and city fire and police units "are assessing damages to buildings, homes, and vehicles primarily in the southern part of the city."

Cluck issued a disaster declaration for his city, which opens the way for federal and state aid.

The town of Lancaster, which also issued an emergency declaration, ordered a a dawn-to-dusk curfew and allowed residents to enter a heavily damaged blocked-off area only with police escort.

"Early assessment reports approximately 300 structures have been damaged; approximately 150 of these structures were severely damaged. At this time no fatalities have been reported," Alicia Hills-Oyedele with the city of Lancaster said in a statement.

The storm system "included damaging hail that ultimately caused hundreds of flight diversions and cancellations," the Dallas-Fort Worth International Airport said in a statement.

As the storm approached, airport officials told passengers to move away from glass windows and take cover in terminal storm shelters.

More than 110 aircraft "sustained varying degrees of hail damage during the storm. Airline personnel are conducting individual aircraft assessments to determine the extent of the impact," the airport said.

More than 400 departures were canceled, while another 40 incoming flights were diverted, though many were expected to return later in the evening.

Airport personnel helping stranded passengers by distributed cots, blankets, pillows and toiletry kits when needed.

Airport teams "are inspecting the airfield for any possible damage or debris, there are no reports of any issues at this time," the statement read.

The airport is the main hub for American Airlines and its regional carrier, American Eagle. The airline said that its airport operations were canceled through through this evening.

Lieutenant Governor David Dewhurst warned Texans to be prepared that more bad weather could come.

"We're in the tornado season, so please ladies and gentlemen, pay attention and if there are any warnings ... go inside room, ground floor and be safe, please," he said.