SAN DIEGO -- A wooden structure covering a pedestrian walkway collapsed alongside an East Village affordable-housing construction site Thursday, injuring 16 passers-by, three critically, police said.

The protective framework on the south side of the 1500 block of Imperial Avenue in downtown San Diego fell for unknown reasons about 12:20 p.m., winding up partially in the street, according to police.

Medics took three people -- a 50-year-old man and two women, ages 57 and 60 -- to a trauma center for treatment of life-threatening injuries, said Maurice Luque of the San Diego Fire-Rescue Department.

A dozen others were taken to hospitals with moderate to minor injuries, and one mildly hurt patient declined to be transported, Luque said.

All of the victims -- 11 males and five females ranging in age from 16 to 64 -- were pedestrians walking or loitering underneath the structure when it crashed to the ground, according to Luque.

A witness named Jose was riding a bus in the area and saw the incident unfold. He told 10News that the scaffolding was "weaving" and then rushed off bus after he said the scaffolding pushed away from wall and fell onto a crosswalk with pedestrians.

People walking under scaffolding had it fall on them, with some being trapped, Jose told 10News.

Jose said one woman was pinned in the wreckage and suffered what appeared to be a broken leg. He also said one man with suffered a serious head injury and he assisted in lifting a 2x4 that had fallen on a woman's neck.

State occupational-safety investigators were called in to determine the cause of the accident.

A short time after the collapse, police arrested a man who ignored officers' orders to stay clear of the area, SDPD Detective Gary Hassen said.

The builder of the low-income project, Allgire General Contractors Inc. of Carlsbad, closed down the site immediately following the collapse, a company official said.

"We are, first and foremost, concerned about the condition of the injured," said Kathryn Musso, operations manager for the firm.

Musso said it was unclear how long the stop-work order would remain in effect.