Carlos Arredondo ran across Boylston Street, jumped the security fence and landed on a sidewalk smeared in blood. In front of him, two women lay motionless. Another woman walked around in black-powder smoke, looking down at the fallen bodies.

“Oh, my God,” he said she repeated, dazed. “Oh, my God.”

Arredondo had been a Boston Marathon spectator, carrying a camera and a small American flag. He dropped the flag. He took four pictures — focusing on a young man crumpled on the sidewalk. The man had a blank expression, and a leg that was only bone below the knee.

Then Arredondo put the camera away. He asked the injured man his name.

“Stay still,” he remembered saying, in accented English. “The ambulance is here.”

In the moments after Monday’s bomb attacks, there were bystanders who defied human instinct — and official orders to evacuate — and ran toward the smoke, instead of away.

There was a Kansas doctor who ran back to help after completing 26.2 miles. A District native who ran down from a post-race party to apply tourniquets. A couple who tried to stop a stranger’s bleeding with a wad of coffee-shop napkins.

And, most astoundingly, there was Arredondo — a man once so broken by grief that his breaking made national news.

First, his son died in Iraq. Then, when Marines came to tell him so, Arredondo set himself on fire inside the Marines’ van. Then, years later, as he was healing, his other son committed suicide.

But Monday — for some reason — when the bombs went off, the broken man came running.

“I did my duty,” Arredondo said the next morning.

In the aftermath of Monday’s explosions, much of the early lifesaving was performed by amateurs: Boston cops, marathon volunteers, plain old bystanders. They tied tourniquets and carried away the injured in wheelchairs or in arms.

On Tuesday, local hospitals said this work — along with the efforts of professional medics on the scene — probably saved lives.

“Tourniquets are a difference-maker. Tourniquets can save a life,” said Joseph Blansfield, a nurse practitioner and program manager at the Boston Medical Center trauma unit, which saw a large influx of patients from the scene. “They proved their value yesterday.”

The man in the wheelchair

Arredondo became the face of this bystander heroism after news photos showed him pushing an injured man down the street in a wheelchair. At the time of the first explosion, he was on the opposite side of Boylston Street, close to the finish line. He had come to support a group of military service members who were marching the race with heavy rucksacks on, as a memorial to fallen soldiers.

One was marching for Alex Arredondo, who was killed in 2004 in Najaf, Iraq. Carlos Arredondo and his wife, Melida, were waiting for that runner. They never saw him.

“That was a bomb,” Arredondo said he thought as soon as it happened. Soon, he arrived at the side of the man without a leg. So did another bystander, who seemed to know what he was doing.

The other bystander asked for tourniquets. Arredondo said he tore pieces off a sweater he had found on the ground.

While the other man tied them on, Arredondo talked to the victim and tried to block the man’s view of his own legs. A native of Costa Rica, Arredondo had some training in this situation — he had been a firefighter and helped to rescue gored bullfighters in the ring.

“You’re okay,” he remembered saying to the injured man on Monday. “Relax.”

Somebody else appeared with an empty wheelchair. An angel, Arredondo thought later. Arredondo put the injured man in the seat. The man had ash in his hair. They wheeled him away, bypassing the medical tent. The man was too badly hurt for that.

“Ambulance! Ambulance! Ambulance!” Arredondo said he yelled.

As they went, one tourniquet slipped off. The blood flowed again. Arredondo grabbed the tourniquet and wrenched it tight. Finally, they found an ambulance.

“What’s his name?” the medic asked Arredondo. Arredondo had forgotten, he said. He asked the man again. Somehow, the wounded man was still calm enough to start spelling it out, to be sure they got it right.

The ambulance doors closed. The man was gone. So what was his name?

“I can’t remember,” Arredondo said Tuesday. He didn’t know what became of him.

The man was later identified as Jeff Bauman Jr. in an Associated Press report. Bauman, 27, lost both legs, the report said.

On Tuesday, Melida was taking his phone calls in a rowhouse in the Roslindale section of Boston. The messages filled up a page: Katie Couric. Fox News. A Boston police detective. Police later took away Arredondo’s clothes as evidence and looked at his pictures from the scene.

Melida had been frozen with fear when the bombs went off Monday. After the chaos subsided, the couple found each other at the Boston Public Library and went home. Melida couldn’t get warm until 2 a.m. The weather was mild for a Boston spring, but the problem wasn’t the weather. she tried to explain why her husband acted so differently from her — and from his own past habits.

“Having lived through the death of Alex, when his reactions were very different . . .” she started to say.

There was a picture of Alex Arredondo in a corner of the room, wearing a Marine dress uniform in a casket. Carlos Arredondo gained national attention for his breakdown after Alex’s death. The Marines who had come to notify Arredondo pulled him out of their van and helped put out the fire. But Arredondo was left with painful burns on his legs.

How did he explain his actions this time?

“You have to get out of that shock” that comes with tragedy, he said. You have to act. “In this case, my instinct was to be a humanitarian.”

Others who helped

In the chaos around him, others had been doing the same thing. In an alley near the finish line, a pizza chef turned his apron into a tourniquet for a woman whose leg had been shattered.

Elsewhere, Bruce Mendelsohn, 44, was three floors up at a post-race party for his brother. Then he heard the explosion, and he knew from Army service what it was. What now? Mendelsohn had worked at the National Law Enforcement Officers Memorial in Washington. He knew what a cop would do in this situation.

“I thought to myself, ‘Well, what would those guys say if I ran away?’ ” Mendelsohn said. He ran downstairs and helped at least four people: A woman with a mangled calf. A mother with shrapnel wounds who was looking for her son. A man and a woman, both badly injured, holding each other in shock.

After 12 minutes or so, it was all over. There was nobody left to help. “I got back to my office,” Mendelsohn said. “I threw up.”

Nearby, Chris Rupe — a general surgeon from Salina, Kan. — had finished the marathon 30 seconds before the blast. He ran away for a while, to be sure there would be no more explosions. Then he came back, talking his way past police. “I told them I was a doctor and I’d like to help.” He helped triage the wounded in a medical tent, still wearing his running gear.

Farther away, a few blocks from the finish line, Kurt Mias and his girlfriend, Jessica Newman, came out of a coffee shop and saw people fleeing toward them. One woman approached, crying, with her lower legs covered in dime-size shrapnel wounds.

“What the [expletive] is happening? Who the hell did this?” Mias, 28, remembered her saying, over and over. Newman, 32, ran into a nearby coffee shop and shoved someone away from the napkin dispenser. They needed a towel, but these would have to do. They pressed the brown napkins against one of the stranger’s legs.

When the woman seemed to be in good hands, they left — passing other wounded people being attended to on the street. An elderly man lying prone. A teenage boy, with a chunk of something embedded in his leg. Before they left, they helped marathon volunteers disassemble a huge area where water and Ga­tor­ade had been set up for the finishers. They needed to clear the road so ambulances could pass.

“Pretty soon, there were almost too many people” trying to help, Newman said. They were hurling whole cases of water bottles out of the way, but there were so many bystanders trying to help that there was no space free to throw them. “You didn’t want to throw it on someone.”

Finally, they left. On Tuesday, retelling the story in a word-jumbled rush, Newman recalled that just an hour before the blasts, she had told Mias, “This is the perfect day.”

“I’m never saying that again,” she said.

Julie Tate and Alice R. Crites in Washington contributed to this report.