Two people, including a mother and one child, were in serious condition at area hospitals, officials said.
Six others, including a mother and her four children, were treated and released, said Sharon Emanuelson, a spokeswoman for Kishwaukee Community Hospital in DeKalb. The other person injured was in good condition.
Another person refused medical treatment, said DeKalb Fire Chief Lanny Russell.
The demonstration, part of a monster truck tour sponsored by NAPA Auto Parts, featured the truck driving over and crushing four cars.
"After its third or fourth attempt of rolling over the cars, and getting back down to the street, it appeared to have lost control and at that point careened off to the left into part of the crowd that was watching the event," said City Manager Mark Biernacki.
After plowing through the crowd about 2 p.m., the truck went through a fence and stopped on railroad tracks, Biernacki said.
Witness Patrick Sheridan, 16, said NAPA staff members had asked observers to stand back from the road after the truck's first few passes.
"I watched the guy go over two or three times getting some air and one time he went up and he came really close to hitting people," Sheridan said, adding that the truck appeared to come within just three feet of bystanders.
The final pass "was like a full-out jump because he, like, floored it and it just went up and landed on the back tires and kept going," Sheridan said.
"It sounded like he couldn't get the gas off and like it kept going and going and going," he said.
The 44-year-old truck driver did not appear to be injured, Biernacki said.
"There was just this sound of steel crunching," DeKalb resident James Vesely told The Daily Chronicle of DeKalb. "I ran over to see if it hit anybody's car, and I saw a woman and a little girl around 3 years old lying on the ground with dirt on them."
Jerry Nix, a spokesman for NAPA's parent company, Genuine Parts Co., said he could not comment. A message left at the auto parts store near the accident site after business hours was not immediately returned.
The city of DeKalb gave the store permission to close the street for the demonstration, Russell said. DeKalb police would not immediately comment.
The manager of a bicycle shop on the block where the accident happened said he didn't see the incident but saw the truck performing stunts beforehand, and estimated at least 100 people were watching.
"It looked very precarious," said Tobie DePauw, manager of North Central Cyclery.