SIMI VALLEY, CA (NBC) - Dozens of Fourth of July celebrants were injured Thursday night when fireworks exploded into a crowd at a Southern California park.

Simi Valley authorities called the detonation a "fireworks mishap," and it appeared that a platform to hold the fireworks had given way. 

"For some unknown reasons the structure that holds these ordinances collapsed and caused them to be firing into the crowd," Simi Valley police Cmdr. Stephanie Shannon told The Associated Press.

The injury count changed multiple times over the course of a quickly developing breaking news story. As of 11:30 p.m., 28 people were reported injured, Ventura County Fire Department spokesman Mike Lindbery told reporters at the scene.

Twenty of the victims were transported to the hospital, four of whom suffered "moderate to severe" injuries and 16 with minor injuries, Lindbery said. Eight people were treated and released on the scene.

Those hurt ranged from young children to the elderly, with injuries that included burns and cuts from flying debris, and some broken bones, Ventura County Fire Department officials told NBC News early Friday. None of the injuries were life threatening.

Between 8,000 and 10,000 people had been expected to attend the Independence Day celebration Thursday night. The crowd was kept at at least 350 feet from the fireworks mortars, the distance required by fire code, Lindbery told NBC News.

Witnesses said the professional fireworks show lasted just a few minutes before the huge explosion went off.

"We saw about three minutes of the show. There was some that went up in the area and everything just kind of scattered outwards and then everybody just started running," a witness told NBC4.

"Everything exploded and parts came flying everywhere," another witness told NBC4. "People were running everywhere. So, it was pretty scary."

It happened about 9:20 p.m. PST at Rancho Santa Susana Community Center and Park, said Cmdr. John Parks, a watch commander for the Simi Valley Police Department.

Parks called the detonation a "fireworks mishap." He said initial information suggested the explosion was accidental.

Emergency medical and fire personnel were already at the park to monitor the fireworks show, so the victims were attended to quickly, Parks said.

Initial injury counts reported 20 people were hurt then that figure was revised to 14 before Lindbery detailed that 28 were hurt in the explosion.

Most of the fireworks were not launched and remained in their mortars unexploded, he told NBC News.

The park was evacuated after the blast, Parks said. Aerial footage showed dozens of people lingering on the outskirts of the scene and near the triage area set up on the grass.

Simi Valley is located about 41 miles northwest of Los Angeles.