Authorities say construction workers accidentally hit a gas line, causing the downtown explosion.
Capt. Len Smallwood of the St. Cloud Police Department told CBS News Station WCCO-TV in Minneapolis that rescue dogs are trying to locate people in some of the buildings leveled by the blast.
Several buildings were leveled by the blast, which happened in the downtown Courthouse Square.
Police and fire officials evacuated at least a six-block area, and the smell of natural gas was in the air. St. Cloud is about 70 miles northwest of Minneapolis.
The blast leveled the rear half of a bar, hurled debris up to a block away, shattered windows of a car driving by, blew out the front windows of the bar and knocked out some power in the area, a local radio station reported.
"I got out of my car and shut my door and all of a sudden, a loud bang went off," said Paul Lutgen, who works about two blocks away.
Lutgen said it sounded like loud fireworks.
"I just saw a huge cloud of smoke rising up by the courthouse," he said.
Gov. Arne Carlson arrived in St. Cloud on Friday afternoon to survey the damage.
"It looked like someone dropped a bomb on the whole area," Carlson said.