By Benno Groeneveld

MINNEAPOLIS, Aug 2 (Reuters) - At least seven people died in the rush-hour collapse of a major interstate bridge over the Mississippi River in the U.S. state of Minnesota on Wednesday.

Officials expected the death toll to rise once recovery operations resumed at daybreak.

Sections of the Interstate 35W bridge broke loose about 6:05 p.m. CDT (2305 GMT), plunging into the river, on to its banks and another roadway. Crumpled vehicles, huge slabs of concrete and twisted steel littered the area.

"Obviously this is a catastrophe of historic proportions," said Minnesota Governor Tim Pawlenty.

Witnesses said a frightening rumble came from the bridge, followed by a thunderous roar, before the 500-foot (160 metre) span came down. Survivors also told the Minneapolis Star Tribune the bridge began to buck up and down.

"I'm lucky to be alive," Marcelo Cruz, 26, said repeatedly.

Cruz had to steer his van into a concrete rail to keep his vehicle from diving off a broken end of the bridge, which stood about 65 feet (20 metres) above the Mississippi.

A witness said she saw half-submerged vehicles and people swimming in the water seeking safety.

Fire Chief Jim Clack told reporters seven deaths had been confirmed and the number would probably rise.

About 20 people were missing when the search was suspended on Wednesday night because darkness posed a danger to divers.

Five miles (8 km) of the Mississippi, the longest U.S. river and a key transportation route, had to be closed on either side of the collapsed bridge, said a spokesman for the U.S. Coast Guard in St. Louis, Missouri.

Sixty people were taken to hospitals, authorities said.

A school bus carrying about 60 children had a narrow escape, remaining intact despite the destruction around it. All those on board were saved, although 10 had to be taken to hospital.

The cause of the collapse was not yet known but the U.S. Department of Homeland Security in Washington said there was no indication of terrorism.

Crews had been working on the bridge, which was built in 1967, to repair the surface, signs and guard rails. The bridge had been inspected in 2005 and 2006 and the Minnesota Department of Transportation found no structural defects.

The department said about 200,000 vehicles a day used the eight-lane roadway. (Additional reporting by Todd Melby in Minneapolis and Diane Bartz and Andy Sullivan in Washington)

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