Mexico City (CNN) Mexico's biggest earthquake in more than three decades brought parts of the capital to its knees on Tuesday afternoon. But moments later, as the earth continued to tremble, many of Mexico City's residents stood up and mobilized.

Volunteers cordon off an area of Mexico City's Coapa district to allow emergency vehicles to pass through.

Volunteers call out to others to locate adrenaline in the donated medical supplies.

Medics from the Mexican Red Cross stand by with a stretcher.

When news emerged that an elementary school had collapsed, city residents and people from surrounding regions -- including some hit hardest by the quake -- traveled to the city's Coapa district en masse, bringing everything and every skill they could muster to help dig out those buried under the rubble . At least 273 people died in the 7.1-magnitude quake.

Sixteen-year-old Luisa and her three friends had been volunteering at the site since Tuesday afternoon. On her way to deliver candy and water to rescuers, she paused for a moment describing the harrowing scene inside.

"It's horrible. We saw a lot of children dead on the floor. Their moms crying," she said.

By Wednesday morning, rescue teams had found the bodies of 21 children and 4 adults who had perished under the weight of the fractured Enrique Rébsamen school.

As the depths of the disaster became clear, an arsenal of volunteers grew, bringing the remaining rescue attempts into sharp focus. Three more people were still missing -- and thought to be alive.

Inside the medical tent on the school site, volunteer doctors are briefed on the progress of the rescue effort.

The city and its residents organized as best they could, sharing the collective sorrow as they mustered all their might for the youngest victims, children who had had become everyone's children.

Throughout the day, an eclectic group of civilian volunteers had descended on the streets surrounding the school in a grassroots relief effort executed with urgency, professionalism and respect.

Volunteers raise their hands asking for silence as first responders listen for signs of life in the debris.

Pickup trucks and motorcades packed with rations of bottled water, food, medicine and blankets pushed their way through the city's notorious traffic with pace.

An army of primarily teenage volunteers acted as traffic wardens in the area around the site, shepherding supplies toward the center of the rescue operations while passing out face masks.

Medics and volunteers sift through donated medical supplies near the school.

Closer to the school, on the main thoroughfare División del Norte, volunteers linked arms in a human perimeter to keep the street open so ambulances and vehicles carrying supplies could make their way to the school.

Rescue team Los Topos ("The Moles"), members of the Mexican navy and marines and a group of volunteer doctors were working around the clock inside the compound.

Emergency rescue workers rest inside the school complex late Wednesday night.

Taking a break from the collective chaos was 70-year-old volunteer Héctor Méndez.

Standing in a half-torn orange jumpsuit at dusk, the veteran relief worker said he had been commemorating the anniversary of the 1985 earthquake just as this one unfolded.

When he heard children were hurt, he, like so many, rushed to the site. While at the scene the day before, he had found four bodies.

Héctor Méndez has dedicated his life to helping others.

Méndez, who has worked in disaster zones all over the world including Haiti, Japan and most recently Florida, said the rescue had been an intense challenge so far. Entering the site, he said, is like "crawling inside like a lizard in a hole."

Volunteer doctors wait inside the medic text as the rescue operation continues inside the ravaged school.

Sergio Romo, a neurologist from Mexico City, volunteered in support of his longtime friend, the school's proprietor.

But Méndez was still optimistic, saying that he was proud of how his fellow countrymen had responded to the scene, and that optimism grown from pain could guide Mexicans through a new chapter.

"Society changed in 1985 after that earthquake. It was a kind of cleaning. Because suffering cleans your spirit."

"So Mexican society now is a kind of catharsis -- kind of a social catharsis, you see."

The citizen-led operation following Tuesday's quake made no distinction between gender, race, age or class. They weren't there for any reason other than to help.

Rain lashes down on a troupe of volunteers outside the school complex as rescue workers inside intensify their search for the missing.

A first responder gazes past a truck filled with materials to aid the rescue effort.

When rescue teams shouted the words "beds," "stretchers," "medicine" and "water," an assembly line of volunteers was quick to answer the request. When rescuers raised fists to quiet the crowd so that teams could hear sounds from the trapped, volunteers followed suit.

Machine operator and rescue volunteer Ivan Ramos, 34, said he's always been one to lend a hand when he can. He had volunteered at Ground Zero after the attacks on the twin towers in New York City in 2011.

Ivan Ramos said: "I have the feeling to help because it's a lot of kids. We want to save them."

Ramos was also worried about the safety of his child, who lives in a neighboring state, and he said he was struck by how the parents must have been feeling.

"It could have been my child -- anyone's," he said somberly.

As the rain poured down early Thursday, volunteer rescue workers kept working their way through the rubble, painstakingly removing pieces of debris from the site of the former elementary school with a surgical unit's precision, a military battalion's efficiency and the love and caring of a parent.