MINNEAPOLIS, Aug 1 (Reuters) - A woman sobbed against an embankment near the collapsed Minneapolis bridge, comforted by strangers and a priest. She told them she had been talking on her cell phone with a friend who was driving on the span when the call was suddenly interrupted.

"She can't talk," one of the men said, explaining the woman had rushed over to the bridge but could not find her friend.

Martha Robeson was also crying, worried that her two granddaughters had been hurt or killed in Wednesday's rush-hour collapse of the 500-foot (160 metre) span over the Mississippi River.

"I want my grandbabies. That's all I want," the 39-year-old said of the 6- and 4-year-old girls who had been on a city-sponsored swimming trip.

Witnesses described a frightening rumble, then a thunderous roar, as the 40-year-old steel and concrete span buckled and collapsed, crashing 65 feet (20 metres) into the river.

Cars were crushed under huge slabs of concrete, flipped onto their roofs or thrown in the river as the bridge, packed with vehicles in bumper-to-bumper commuter traffic, went down.

Leif Hanson and a friend were bicycling nearby and rushed to the tumultuous scene. "There was a truck below, having explosions," he said.

His friend Andy Schwich said he saw rescuers pull up a lifeless body, likely one of at least half a dozen people killed in the unexplained collapse of the bridge. At least 20 people were injured.

On a refurbished stone railroad bridge parallel to the collapsed span, hundreds of cyclists and pedestrians stopped and stared on an unusually warm summer evening.

After the ambulances had left, floodlights were set up, tow trucks lined an adjacent avenue, and an oversized two-story-high backhoe moved into position to begin cleaning up the wreckage.

Down in the river, a single rescue boat searched vainly amid a tangle of steel girders and chunks of concrete, a few cars submerged just beneath the surface. By nightfall, some 50 cars had been searched by rescue teams.