At least three people were being treated for injuries, hospital officials said, and more ambulances were arriving. The Sequoyah County Sheriff's Department and the Oklahoma Highway Patrol said they were in a state of emergency.
Several cars were in the water, and dive teams were on the way to the scene, in the southwest part of the state, said Sam Graves, communications officer for the Highway Patrol.
A second bridge, at Webbers Falls, where traffic would have tried to pass after the first bridge collapsed, also was hit Sunday morning and was shaking but hadn't collapsed, Webbers Falls Mayor Jewell Horn said. She said that bridge had since been closed.
Horn said the bridge that collapsed was about 20 years old. She said it wasn't clear precisely when the bridge was hit. It could have been hit before dawn and later collapsed under the weight of the vehicles, she said.
The area is about 100 miles east of Oklahoma City and about 35 miles west of the Arkansas state line.
Chad Wetz of Muscogee Medical Center said at least three patients had been taken to his hospital, two were in stable condition and one was in critical condition. One is a 62-year-old man, and two are in their 30s or 40s, he said.
One patient told him he only recalled the bridge collapsing, Wetz said.
"He said he was driving along, and the next thing he knows, there was no pavement under him, and that he was headed for a concrete pillar of some sort and hit the water," Wetz said.
Horn said it's possible that seven tractor-trailers and nine vehicles went into the water during the heavy downpour.
"This barge came loose. It was an accident. It hit the bridge and the bridge collapsed and that was it," Horn said.