This view shows the Interstate 35W freeway bridge over the Mississippi River after it collapsed, August 1, 2007, in Minneapolis, as emergency crews gather. The bridge collapsed during evening rush hour sending vehicles, tons of concrete and twisted metal crashing into the water. [AP]

Minneapolis - Six people were killed when a highway bridge collapsed and hurled vehicles into the Mississippi River in central Minneapolis during evening rush hour on Wednesday, the city's mayor said.

"We have confirmed it will be a very tragic night when it is over," Minneapolis mayor R.T. Ryback told a news conference.

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

The entire span of the interstate bridge collapsed into the Mississippi River, sending vehicles, tons of concrete and twisted metal crashing into the water.

The Interstate 35W bridge, which stretches between Minneapolis and St. Paul, was in the midst of being repaired when it broke into several huge sections.

The Homeland Security Department had received no indications Wednesday night that the collapse was related to an act of terrorism, department spokesman Russ Knocke said in Washington.

"We continue to monitor the situation. At this time, there's no indication of a nexus to terrorism," Knocke said.

A burning truck and a school bus clung to one slanted slab, while at least eight cars and a truck were submerged in the river.

The bus had just crossed the bridge before it crumpled into pieces, and broadcast reports indicated the children on the bus exited out the back door.

Ramon Houge told the St. Paul Pioneer Press that he was on his way home from work on the bridge when heard a rumbling noise, saw the ground collapse and cars go down.

Traffic was bumper to bumper and hundreds of people would have been involved, he said. He said cars backed up 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.

Dozens of vehicles were scattered and stacked on top of each other amid the rubble. Some people were stranded on parts of the bridge that aren't completely in the water.

Local television stations captured video of injured people being carried up the riverbank. There was no official word on injuries, but dozens of rescue vehicles were there. Divers were also in the water.

Gregory Wernick Sr., Rockford, Ill., 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 10 minutes before it happened," he said. "That's just too close to call."

He was standing about 200 feet away on top of a parking ramp with large group of people.

"I've never seen anything like this," he said.

The Homeland Security Department had received no indications Wednesday night that the collapse was an act of terrorism, department spokesman Russ Knocke said in Washington.

"We continue to monitor the situation. At this time, there's no indication of a nexus to terrorism," Knocke said.

The arched bridge, which was built in 1967, rises about 64 feet above the river. Workers have been repairing the 40-year-old bridge's surface as part of improvements along that stretch of the interstate, the Minneapolis Star-Tribune reported on its Web site.