Four Denver police officers on bicycles, returning from escorting hundreds of East High School students marching Wednesday in solidarity with protesters in Ferguson, Mo., were struck by a car and injured, one of them critically.

Run over as they monitored the students, the officers and their mangled bicycles were tossed across the sidewalk lining East Colfax Avenue east of downtown. The car left in its path a trail of wreckage — a helmet, police gear and blood.

The black Mercedes that hit the officers dragged one of them dozens of yards, depositing him critically injured on a curb where he clung to life.

“He was just lying there,” Taylor Ellison, a sophomore at the high school, said of the officer. “His pants were ripped. He wasn’t moving.”

Ellison said the officer was so badly injured he thought the man was dead.

Officer John Adsit, a nine-year veteran, underwent several hours of surgery at Denver Health Medical Center. And although police said he was “not out of the woods,” his condition was stable.

In a statement, Adsit’s family thanked the public for their thoughts and asked for their continued prayers.

Witnesses say the Mercedes’ driver — described as a man in his late 40s — accelerated west through the intersection of Colfax Avenue and High Street into the officers and a group of pedestrians before slamming into an SUV in a Taco Bell parking lot nearby, coming to rest.

WATCH DPTV: Reporter Jesse Paul describes the scene of the crash

“Listening to people say ‘F the police!’ — and then they get hit trying to make sure you aren’t hit by a car,” Ellison said, shaking his head.

The ordeal unfolded before dozens of student protesters and passers-by, who gaped in disbelief at the carnage, many of them still in tears hours after speaking with detectives. One woman dressed in nurse’s scrubs was covered to her knees in blood after apparently coming to the aid of the injured cops.

While witnesses speculated Wednesday about what led to the incident, authorities did not release an official determination. Police Cmdr. Matt Murray said the driver, whose name has not been released by police, had a medical issue and police believe that issue occurred before he struck the officers.

Three officers transported to Denver Health were released in the afternoon. The driver, also taken to Denver Health, was last listed in stable condition.

Police Chief Robert White said it was “not our best day.”

Police said the bicycle officers from District 6 regularly work the 16th Street Mall and all have at least nine years of service, including one with 17 years. Before they were hit, the officers were taunted by some protesters as they rode along the group near the Capitol, peddling uphill and keeping traffic out of the teens’ way.

WATCH DPTV: The owner of Torta Grille describes the crash

“He was honking his horn and waving his hands up” with the protesters before he drove into the officers, said Doug Acton, who was sitting as a passenger in a stopped semi-trailer beside the Mercedes before the incident.

“He gunned it straight at them,” Acton said.

Roberta Martinez was near the officers waiting at a bus stop when the Mercedes came speeding at her. She was able to dive out of the way.

“It sounded like he stepped on the gas and just went out of control,” Martinez said. “I should have been hit. The Lord was watching me. He had to be. I can’t believe I’m still standing here.”

“It was horrifying,” Lily Mogen, an East High senior, said through tears.

East High School students planned the protest earlier in the week, expecting a few dozen participants. On Wednesday morning, their numbers quickly swelled to roughly 1,000 as the young protesters walked out of class in an event that teachers and school officials had been made aware of.

They began marching west on Colfax Avenue to the Capitol with a police escort, carrying signs and a giant “WE ARE FERGUSON” banner, before moving on to the 16th Street Mall. Dozens joined the group, which chanted the mantra of protests associated with the slaying of Michael Brown: “Hands up, don’t shoot!”

The student protest, which came at the same time as a large Sand Creek massacre commemoration at the Capitol, drew dozens of police resources downtown.

About noon, after blocking the intersection of Broadway and Colfax for roughly five minutes, the protest had calmed somewhat, and students were walking in the eastbound lanes of Colfax back toward school.

It was then the officers were struck.

Amarae Moland, a sophomore, was one of students who organized the protest by handing out fliers at school and rallying students on Facebook. They were expecting about 30 kids, not hundreds, and did not plan to march, she said.

“I was really surprised and devastated,” Moland said of the accident. “I didn’t expect anything like that to happen nor want that to happen. We were trying to get our voices out there, and it’s sad that something this tragic had to occur.”

Police were investigating the scene well into the evening, maneuvering around a cobweb of red and yellow police tape and dropping evidence markers on the roadway. East Colfax Avenue did not reopen until nearly 4 p.m.

By late afternoon, the normally bustling area was mostly abandoned except for a few officers who remained watching over the mangled bicycles and Mercedes, somberly keeping watch over the roadway stained with blood.

Staff writers Anthony Cotton and Yesenia Robles contributed to this report.

Jesse Paul: 303-954-1733, jpaul@ denverpost.com or twitter.com/JesseAPaul