The smell of paint wafted through Benito Juárez International Airport. Jackhammers buzzed. Heavy machinery and caution tape blocked hallways.

Officials worked around the clock to finish $500 million worth of renovations to the country’s most important airport before the World Cup opens this week in Mexico City. The challenge: The nearly 100-year-old airport, long cramped, leaky and outdated, had to remain open during remodeling.

More than five million visitors are expected in Mexico during the six-week tournament, which is jointly hosted by the United States and Canada. Airport officials said they expected three million to four million passengers to pass through Benito Juárez, and recently insisted the work would be completed in time.

“We’re 100 percent going to achieve that,” said Adm. Juan Manuel Muñoz Gómez of the Mexican Navy, who helped oversee the airport remodeling ordered by President Claudia Sheinbaum a year ago. “We’re prepared for that amount.”

But the World Cup has exposed Mexico City’s airport infrastructure to uncomfortable scrutiny.

Although passenger-related renovations were finished by May 31, it remains to be seen how the upgraded airport will respond to the increased demand from the tournament. More work is needed, though, and officials said the remaining operational upgrades would resume after the World Cup. But questions about the speed and quality of the work already arose last week when part of a pedestrian bridge fell and blocked traffic.

Experts said the overcrowded facility had been neglected for decades. Nor is it, they argued, befitting of one of the world’s largest cities — a metropolitan area of 23 million people — or of a country of 133 million people with a growing economy.

The airport’s troubles are rooted in a decision made years earlier. In 2018, the country’s president at the time, Andrés Manuel López Obrador, canceled construction of a new $13 billion airport about 15 miles from downtown, even though the project was one-third completed. Instead, he built a $4 billion commercial airport at a military air base twice as far away.

The result is the lesser-used Felipe Ángeles International Airport, which has grown since its opening in 2022, but handled only seven million passengers last year compared with Benito Juárez’s 45 million.

The renovations underway at Benito Juárez are largely cosmetic, said Peter Cerdá, the chief executive of the Latin American and Caribbean Air Transport Association. The airport — which opened in 1928, with its first terminal expanded in 1952 and second inaugurated in 2007 — had reached “a collapsing point,” he said, and the work in recent years has amounted to “band-aids.”

He pointed to cargo operations being moved to Felipe Ángeles and a cap on flights to Benito Juárez, which had exceeded its original design capacity of 32 million passengers per year. The moves drew the ire of the Trump administration, which accused Mexico of artificially propping up Felipe Ángeles and of increasing the costs for American businesses by millions of dollars with the shift of cargo operations.

Mr. Cerdá said he was not concerned about safety. But he worried about the ability to handle projected regional growth of up to 6 percent annually without major work, like expanded or new terminals and runways.

“These are good steps but are short-term solutions for a World Cup,” he said. “That is not a mid- to long-term solution to address the growth that Mexico City and Mexico will see.”

Mexican navy officials, who operate Benito Juárez, insisted that the airport remodeling was more than cosmetic. Yes, the terminals have better décor, improved food options, new flat screens, redone bathrooms, upgraded waiting areas and wider hallways reclaimed from former office and retail spaces. But they said operations had also improved.

Nearly 60 new automated boarding pass readers and more powerful bag scanners, officials said, had cut average security times to seven minutes from 17 minute. Immigration now has up to three times as many interview modules and automatic gates. A new facial-recognition security system covers the airport, and an improved baggage-claim process is in place.

New software aims to better manage the flow of planes, drainage on the airport’s two runways has been upgraded, and new taxiways allow planes to reach gates faster. Those improvements, officials said, helped the airport win recent approval to increase the number of flights per hour to 46 from 44, still well below the peak of 61 in 2022.

“It’s not a matter of just painting,” Adm. Muñoz Gómez said. He added that Mexico would benefit from the renovations long after the tournament ends. “It was a demand of the people,” he said. “It’s something we all need.”

As workers recently laid tile, Roger Limon, 54, waited for his flight to Los Angeles. He visits the airport several times a year and used to live in Mexico City for three decades. Back then, he said, the airport was already too old and too small for such a metropolis.

“It’s so many people,” he said.

Nearby, José Luis Cruz Ovando, 68, waited with his wife for a flight to southern Mexico. He said they had come to Benito Juárez every few months for years.

“They’re putting on makeup, as we say here, to make it look nice,” he said. “And it does look nice. The gates are easier now. But I still worry about the runways and space.”