The final whistle came with a deafening roar that rolled across Mexico City.

Red, white and green smoke engulfed Paseo de la Reforma Avenue as fireworks burst overhead and foam from spray cans fell through the night like snow. More than a million people flooded the boulevard, shoulder to shoulder, waving flags, hugging strangers and chanting beneath the Angel of Independence statue until even the monument disappeared into the smoky haze.

On Tuesday night, the entire country seemed suspended in a rare moment of collective joy after Mexico defeated Ecuador in the World Cup, moving to the round of 16. For the first time in four decades, the team advanced to its fifth match in the tournament, breaking what generations of fans had called the country’s curse.

But, for many Mexicans, what appeared to be an extraordinary display of soccer fandom meant something else — something deeper.

After years in which Mexico had become synonymous with cartel violence and disappearances — more than 130,000 people are officially missing — the tournament has offered a rare reprieve: a reason to fill the streets for celebration rather than fear, grief or protest.

“We needed this after all this time of bad news; we needed a relief from all of those things that we face as a country,” said Ramses Rincón, a college student whose face was painted with the colors of the Mexican flag and who was wrapped in a large Mexican flag. “It is important to disconnect from reality, even briefly, from the uncertainty we live with. But it’s also deeply Mexican to keep finding reasons to celebrate.”

Just months ago, Mexican cities had been filled with burning trucks, blocked highways and gunmen sowing chaos after the authorities killed one of the country’s most powerful cartel leaders and criminal groups retaliated in a show of force.

Now, since the beginning of the World Cup, victories by the national team — known simply as El Tri, a reference to the three colors of the Mexican flag — have sent hundreds of thousands of fans into plazas and boulevards, waving flags and climbing monuments, traffic lights and bus shelters. Fans embrace each other and foreigners alike as if they had known one another for years. Subway cars echo with songs. Rivers of people move not to escape violence, but toward one another.

A duck named Merlin, dressed in a tiny Mexico jersey and custom socks, has become an unlikely mascot. At one game, revelers poured tequila for a rooster. A goat in the national team’s green shirt wandered through the festivities.

Mexico’s improbable run has also awakened a flicker of collective optimism that goes beyond soccer.

The tournament’s unofficial slogan is a simple, question: “¿Y si sí?” which roughly translates to “What if they can?” It has become a national refrain embraced by brands, restaurants, influencers and millions of fans and captures the possibility, however fleeting, that things might finally turn out better than expected.

A leading Mexican producer of chicken, eggs and other food products has begun printing the phrase on its eggs.

But beyond the branding, ¿Y si sí? invites Mexicans to suspend a lifetime of lowered expectations, not only about the national team, but also about the possibility that this time something improbable, something extraordinary, might actually happen.

In a country so accustomed to disappointment, ¿Y si sí? has become an act of faith.

There’s also the rocking.

Crowds surround city buses, police trucks and passing cars, swaying them from side to side in a ritual that has become one of the defining images of Mexico’s tournament. Television correspondents reporting live have been swallowed by chanting crowds, and, sometimes, hoisted into the air.

Attendance in Mexico City, the capital, swelled with each victory by the national team: roughly 400,000 after the opener; 800,000 after Mexico beat the Czech Republic; and more than a million on Tuesday night, according to the local authorities. The crowds were so overwhelming that at least four people died of asphyxiation in Mexico City’s center.

Again and again, fans described the celebrations as a proud display of national character, of what they believe makes their country unique: warmth, spontaneity and irrepressible joy.

Diego Moreno, a street vendor, stood above the crowd on Reforma drenched in foam, shouting at the top of his lungs. Asked to describe Mexican fans, he barely paused.

“This is Mexico!” he yelled. “Mexico is a feeling. It’s pure heart!”

For many, the celebrations were also a chance to show another face of the country.

“Mexico is not just its problems, drug traffickers, insecurity and everything else that plagues us,” said Selene Gómez, a college student. “Mexico is so much more than that.”

The euphoria after Mexico’s victory over Ecuador, and the three that preceded it, has felt almost disproportionate to the actual achievement. Many people recognized that it was also about finding release from the emotional weight of everything that came before. The victories have offered a temporary suspension of the country’s anxieties, a break from the relentless cycle of violence and bad news. Or, as the Mexican writer Juan Villoro put it in a recent column, the national team has given Mexico “a splendid dose of unreality therapy.”

One of the tournament’s biggest points of contention has been the prohibitive cost of tickets, which put matches at Azteca Stadium beyond the reach of many fans.

So Mexicans turned the streets into their stadium.

Tuesday was no different. Hours before kickoff, hundreds of thousands had already gathered, draped in the national team’s green, transforming central Mexico City into something closer to a music festival than a sporting event. Food carts lined the avenues. Music blared. Fans staged impromptu boxing matches and hung piñatas from light posts.

There was something familiar about the spectacle, something that reached beyond soccer and felt deeply Mexican.

In “The Labyrinth of Solitude,” the Nobel laureate Octavio Paz described the Mexican fiesta not as mere diversion but as a ritual of liberation, a moment when Mexicans break free of their customary reserve and surrender to communion, excess and chaos.

“Among us, the Fiesta is an explosion, a bursting forth. Death and life, jubilee and lament, song and howl ally themselves in our festivities,” Mr. Paz wrote in his seminal essay published in 1950. “There is nothing more joyful than a Mexican fiesta, but there is also nothing sadder. The night of celebration is also a night of mourning.”

For Mr. Paz, the fiesta temporarily dissolves solitude into collective abandon. That spirit has found a new expression in this World Cup.

At the Angel of Independence on Tuesday night, Osmar Almora, 11, sat in his mother’s arms, blowing a plastic horn. Raúl Jiménez, his favorite player, had scored. One day, he said, he hoped to wear Mexico’s jersey himself.

“I knew we were going to win,” he said.

Around him, the boulevard echoed with “¡Sí se pudo!” “Yes, it could be done!” For millions of Mexicans, it was a chant about finally believing again.

Osmar, it seemed, had never stopped.