One hundred years ago this week, an epic moment unfolded in Mexico City: The revolutionaries Emiliano Zapata and Pancho Villa rode into the capital on horseback, trailed by their respective peasant armies.

Today, just as a century ago, the country again is gripped by widespread discontent. A growing number of Mexicans complain of corrupt and incompetent rule. Protests and rallies unfold weekly, often more frequently.

Only today, there is no charismatic rebel leader at the fore, neither a Zapata nor a Villa. Without leadership, observers say, the Mexican Spring may end as a fizzle, more Occupy Wall Street than Mexican Revolution, Chapter Two.

Fernando Gamboa Quezada, an economics professor at Mexico’s largest university, peered Saturday from the steps of the Angel of Independence, the iconic obelisk along the capital’s principal boulevard, as marchers clogged downtown in another protest, timed to coincide with the anniversary of Zapata and Villa’s arrival.

“There is a lot of discontent in the country,” he said. “The farmers, the students, the teachers and, I believe, even the government workers are unhappy. Corruption is rampant.”

The last straw for many Mexicans has been the way President Enrique Peña Nieto has handled a probe into the disappearance of 43 students from the Ayotzinapa teachers college in Guerrero state. The students went missing Sept. 26, when police allegedly rounded them up and turned them over to a criminal gang for mass execution.

Distrust in state institutions is so high that the Peña Nieto government has turned over key functions of the criminal investigation to foreigners. A group of Argentine forensic scientists has taken part in the excavation of possible grave sites, and recovered bone fragments were sent to a university in Innsbruck, Austria, for DNA comparison with family members.

Peña Nieto, who presided Monday afternoon over the opening of the Ibero-American summit, an annual regional gathering of heads of state, found signs of isolation even at the summit site in Veracruz. The leaders of Argentina and Brazil stayed home, and the arrival of the Cuban and Venezuelan presidents remained iffy.

A parallel event for first ladies and spouses was canceled, apparently because few spouses wanted to share the stage with Mexican first lady Angelica Rivera, an actress ensnared in an influence-buying scandal with a government contractor who has built a mansion for the couple. Queen Letizia of Spain was among those who chose to stay home.

Mr. Gamboa, the professor, said Peña Nieto still has many ways to deal with unrest and is likely only to be weakened, rather than seriously challenged.

Like other analysts, he noted that protesters in Mexican streets run the gamut from public schoolteachers who resist an education overhaul to professionals seeking greater social and economic mobility. No single leader has emerged to carry the banner.

“Without leadership, it might happen like in Egypt, where there was a spontaneous uprising, and then it fizzled out,” Gamboa said.

Another analyst, Francisco Rodriguez, said discontent may be high, but many disparate factors, ranging from plummeting oil prices to impunity for corruption in the top of the government, fuel the unhappiness, leaving it with little core focus.

“The spark from Ayotzinapa, despite what functional experts of history might say, will not set this pile of straw afire,” Rodriguez wrote Monday on the almomento.mx website.

Perhaps what has contributed the greatest passion to the protests is a broad anxiety that parts of the state itself, particularly the police, are to be feared.

“Everyone of us is at risk of being ‘disappeared,’” said Jaqueline Bermudez, a 23-year-old design student at ICEL University in the capital. “When I see a police officer, I feel more fear than confidence. We don’t know whom to trust anymore.”

For all those who long for a better Mexico, many fear what change might bring. Lawmakers from Peña Nieto’s Institutional Revolutionary Party and two allied factions in the Chamber of Deputies early this month passed a proposal that would empower authorities to ban street marches. The measure has yet to win approval in the Senate.

Peña Nieto warned in mid-November that a hidden hand was determined to destabilize his government, and on Dec. 4 he called on Mexicans “to overcome the pain” of the disappearance of the students.

Industrial tycoons and business leaders worry about the nation’s direction.

“There are many radical groups working at destabilization, at crisis, at everything,” Gerardo Gutierrez Candiani, head of Mexico’s Business Coordinating Council, a leading nationwide business group, told foreign reporters Thursday. “There are criminals, organized crime, guerrillas, many groups in this process. It is very delicate.”

Even if anger over the 43 students dissipates, the pace of disappearances in the country continues at a rapid clip, said Mario Patron, head of the Miguel Agustín Pro Juárez Human Rights Center.

“The official register of disappeared persons on the last day of October stood at 23,605 people,” Mr. Patron told MVS Radio Monday. “Under the current administration, there’ve been more than 5,000 people who have disappeared. . . . We have a generalized problem.”

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And if a political figure were to emerge to harness the desire for change and rule of law in Mexico, it could prove to be a fatal mission.

Neither Mr. Zapata nor Mr. Villa lived many more years to see their revolution unfold. Zapata died in a hail of bullets in 1919, while Villa died in 1923 after a pumpkin seed vendor ran at him shouting “Viva Villa!” In that instant, hidden riflemen shot Villa dead.