A busy four-lane highway bridge that spans the Mississippi River northeast of Minneapolis collapsed during the evening rush hour, sending cars and tonnes of concrete crashing into the water.

At least three people were reported dead and more than 20 injured. Minneapolis dispatched every ambulance in the city to the scene as local hospitals went on a disaster footing.

Three sections of the Interstate 35W bridge collapsed at about 6.05pm local time. A large lorry caught fire, and flames and black smoke billowed into the sky.

The US Department of Homeland Security ruled out terrorism and suggested mechanical failure as the cause of the disaster. The bridge had recently been closed for maintenance work. Workers were repairing the 40-year-old bridge’s surface as part of improvements along that stretch of the interstate highway at the time of the collapse.

An engineering assessment in May last year recommended monitoring of “fatigue cracking” on the girders.

Local television stations showed live footage of injured people being carried up the riverbank. Dozens of rescue vehicles were on the scene and divers were searching through the Mississippi for survivors. Some people were stranded on parts of the bridge that were not completely in the water.

Witnesses said that they heard a rumbling sound as the bridge collapsed. “First I heard this huge roar,” Leone Carstens, a nearby resident who watched the drama unfold from the window of her 18th-floor apartment, said. “I was at my computer. Initially I thought, ‘Wow was that an airplane?’ ”

Ramon Houge, from the neighbouring city of St Paul, was on his way home from work and was driving on the bridge when heard a rumbling noise before seeing the ground collapse and cars go down.

He said cars reversed as best they could and he parked in a construction zone and was finally able to turn around and drive off the bridge. “It didn’t seem like it was real,” he said.

Gregory Wernick drove over the bridge shortly before the collapse. He stopped to get a drink nearby and heard commotion so he went back. “I figure I crossed about ten minutes before it happened,” he said. “That’s just too close to call.” He was standing about 200ft (61m) away on top of a parking ramp with large group of people. “I’ve never seen anything like this,” he said.

Television pictures showed that sections of the road leading to the bridge had also collapsed, in places crushing cars and lorries, some of which were ablaze. About 20 cars on top of each other could be seen in one image, below where the bridge had stood. They were partly buried under rubble and mangled steel. Huge chunks of the bridge jutted out of the river at odd angles, in places surrounded by cars half submerged in the water.

A large, burning lorry and a school bus clung to one slanted slab, while an unknown number of vehicles were submerged. The bus had just crossed the bridge before it failed but local media reported that the children had managed to escape from the bus through the back door.

A lorry driver also escaped uninjured after his vehicle was cut in half. One witness said she saw people in the water.

Police were telling people to leave the area amid fears of several burning vehicles in the area. A firefighter on the scene said that everybody from the north side of the bridge was pulled from the scene alive and that at least 100 vehicles were involved in the accident.

A nursing supervisor at Hennepin county medical centre said: “We have multiple patients. Some critical, some non-critical.”

The road was carrying bumper to bumper traffic when the 500ft steel arch bridge collapsed. The bridge, built in 1967, had stood 64ft above the river.

The Minnesota Department of Transportation told local media that 200,000 cars use the bridge every day.

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