Two truck drivers who were making deliveries were injured, GM spokesman Dan Flores said.
One of the truck drivers was sent to a Midwest City hospital, but the extent of his injuries was unknown, Flores said. The other suffered minor injuries and was treated at the plant.
None of the 1,000-plus employees inside the plant was injured, Flores said.
Gov. Brad Henry said damage to the GM plant will have a significant impact on the state and our economy.
"They are just very, very excited by the fact that nobody was seriously injured," Henry said. "It's miraculous," Robert D. Jones, GM plant personnel director, said of the low number of injuries.
A warning siren sounded at the plant about 20 minutes before the twister hit. Workers took shelter in a concrete-lined room in the plants old paint shop.
Flores said plant officials were having difficulty evaluating the damage without electricity.
GM spokeswoman Kathy Oden said the plant will not produce vehicles today, but some employees will be securing the dam aged plant.
At a United Auto Workers union hall just west of the GM plant, about two dozen people were attending a pre-retirement seminar when the tornado struck.
"It sounded like gravel hitting a window and plywood being torn up," Lori Johnson, 42, of Oklahoma City, said.
The leader of the seminar was monitoring a TV broadcast as the tornado approached.
"He told us to get down. He said 'here it comes,' and we heard it coming," said seminar participant Leola Goodridge, 63, of Ok lahoma City.
"The roof, everything came off," Art Henry, 64, of Bridge Creek, said.
Debris blowing through the room cut his arm, which was later wrapped with gauze.
He also suffered scratches to his head. Seminar participants said they covered their heads and crouched in two rows one on each side of the room as the tornado roared overhead. When the debris set tled and the winds calmed, they looked up and were surprised to find a large steel vehicle bumper lying on the floor between them. The room they were in survived, but other parts of the building were destroyed.
Downed power lines were everywhere in the area, with one blocking the southwest entrance to the GM parking lot. The worst damage at the GM plant was to the new paint shop, a $277 million structure that opened in the spring, Jones said. It sustained severe damage to its west wall.
The body shop, power house and a couple of cooling towers also sustained damage, he said.
Only skilled trades workers and housekeep ing should show up this morning and enter through the main security gate, he said.
More than 3,000 people work at the massive 4-million-plus square foot plant that sits on 430 acres in southeast Oklahoma City. The plant produces 608 vehicles a day.
The night shift begins at 4:30 p.m. and more than 1,000 people were at work when the tornado hit, a company official said.
The plant reopened in January 2002 after an eight-month, $700 million retooling to switch production from passenger cars to sport utility vehicles.
CONTRIBUTING: Staff writers Randy Ellis and Bryan Dean