OAKLAND — Twelve workers, some left trapped knee-deep in wet concrete and others dangling from scaffolding, were injured Friday morning when a building under construction in the Pill Hill area partially collapsed, authorities said.

Workers were in the process of pouring concrete on the second floor when scaffolding collapsed, causing some to fall more than 10 feet and land in “quicksand-like” fresh cement, officials said.

Some were able to get to safety on their own and others were assisted by firefighters. When emergency crews arrived, workers were using shovels to dig their colleagues out of the wet concrete. Footage from a news helicopter showed firefighters tending to at least three workers, two of whom were strapped to stretchers.

The injured were taken to hospitals for cuts, bruises and strains, but no fatalities or major injuries were reported.

Oakland Fire Battalion Fire Chief Ian McWhorter, who was still cleaning wet cement off his boots almost four hours later, said earlier the on-site workers “did an excellent job of extricating” their fellow workers before firefighters arrived and took over.

McWhorter said the cement was “kind of like quicksand” and rescuers used plywood and planks to reach trapped workers so they would not sink.

The collapse was reported about 9:34 a.m. on Hawthorne Street between Broadway and Webster Street near Summit Medical Center.

What caused the structure to fail was not immediately known.

The site, 3093 Broadway, is the former Bay City Chevy dealership and slated to become 423 wood-frame apartments over two stories that include concrete podiums and retail space on the ground floor, according to the website of the general contractor, Johnstone Moyer Inc. A person who answered the phone at the San Carlos-based company declined to comment.

A spokeswoman for the developer, CityView, did not immediately have information.

The project is part of a larger plan to turn the old Auto Row on Broadway into a neighborhood with homes and retail. The construction site abuts Sprouts Market.

Jerry Ogden was on the 10th floor of the nearby medical center when the collapse happened, and said he and others “felt a small shake. Everyone assumed it was a small earthquake and we just went about our business.”

CalOSHA representatives were on the scene as well as city building engineers to determine what caused the collapse.

Numerous streets in the area were shut down during the rescue operation.

Check back for updates.

Staff writer David DeBolt contributed to this story.