Fifty years ago on Monday, a violent coup ended one of Latin America’s most stable democracies, brought an abrupt halt to the Chilean military’s tradition of noninvolvement in politics and ushered in 17 years of ruthless dictatorship.

Salvador Allende, Chile’s socialist president, had embarked on an ambitious agenda that included the nationalization of the copper industry, land redistribution and state control over other strategic industries and banks.

As the economy spiraled out of control and political polarization fueled increasing violence, businessmen, conservative politicians, professionals and some trade groups pressed for military intervention.

The civilian and military conspiracy to overthrow Mr. Allende’s government, aided by C.I.A. financing and covert operations to destabilize the country, culminated in a bloody coup, the likes of which Chileans, unlike people in other Latin American countries, had never experienced.

Decades later, the left and the right still exchange blame for the breakdown of democracy. Far from the promises of “never again” pronounced by some military leaders, some on the right justify the coup and play down the human rights violations that followed. This month, right-wing opposition leaders refused to sign a government-sponsored commitment to democracy and declined to participate in official anniversary events.

On Sept. 11, 1973, Mr. Allende refused to leave the presidential palace, despite threats from the Air Force that it would be bombed if he didn’t surrender. The army and police had already gained control of downtown Santiago, the capital. The Navy had put the coup in motion early that morning in the coastal port of Valparaíso, while the Air Force dropped rockets over a handful of leftist radio stations in Santiago.

By noon rockets had fallen over La Moneda, as the palace was called, and fire quickly spread across its wooden floors, ceilings and beams.

Dozens of advisers, doctors, government ministers, secretaries, detectives, personal bodyguards and two of Mr. Allende’s daughters stayed by his side. Some cabinet members set out on foot to negotiate with the military and were arrested. Mr. Allende secured a brief truce to allow some people to leave. The president’s bodyguards and Mr. Allende himself attempted to fight off the uprising, but it was clearly futile.

Mr. Allende, a physician by training but a career politician and a member of Congress for 25 years, was democratically elected in 1970. The morning of the coup, before the last radio station loyal to the president was silenced, he delivered a somber farewell: “These are my last words, and I am certain that my sacrifice will not be in vain. I am sure that, at least, it will be a moral lesson that will punish the felony, the cowardice and the treason.”

Hundreds of government officials, political leaders and local leftists were later sent to Dawson Island, a remote spot in the Strait of Magellan, where they endured forced labor and torture.

High-ranking military and police officers had been plotting to overthrow the president for months, but General Augusto Pinochet joined the conspiracy just two days before.

He had risen to army commander in chief weeks earlier, replacing Gen. Carlos Prats, who was known for his adherence to the Constitution and was forced to resign. General Pinochet, who had kept his political opinions to himself, was also regarded as a “constitutionalist” officer and had Mr. Allende’s support.

In September 1974, under General Pinochet’s orders, according to a judicial investigation, the regime’s secret intelligence agency, known as DINA, killed General Prats and his wife by placing a bomb under their car in Buenos Aires, Argentina, where they had gone into exile.

Although the military junta had agreed to rotate its leadership, starting with General Pinochet, in 1974 he elevated himself to the until-then-nonexistent rank of “captain general,” and proclaimed himself supreme leader of the nation and president of the republic.

For nearly 17 years, General Pinochet concentrated all political and military power and was ultimately responsible for the widespread human rights violations unleashed by the armed forces and clandestine intelligence agencies under his control.

More than 2,000 dissidents died under torture, were executed or were killed by other means, and nearly 1,500 disappeared, according to reports issued by Chile’s government.

Most of the deaths or disappearances of supporters of Mr. Allende took place in the months after he was overthrown, but political repression continued until military rule ended. In August, Chile’s president, Gabriel Boric, a leftist and an admirer of Mr. Allende, authorized a plan to search for the 1,469 people who are still missing.

While there is no evidence of direct American involvement in the coup, the Nixon administration made sure to “create a coup climate” as soon as Mr. Allende won the presidential election on Sept. 4, 1970, according to declassified U.S. documents.

Eleven days later, President Richard M. Nixon met with Richard Helms, the C.I.A. director, and ordered the agency to carry out covert operations to impede Mr. Allende’s inauguration.

Handwritten notes by Mr. Helms of that meeting revealed the president’s instructions: “One in 10 chance, perhaps, but save Chile”; “not concerned risks involved”; “full-time job — best men we have”; “10 million available, more if necessary”; “make the economy scream.”

The plan involved a propaganda campaign, bribing members of Congress and instigating a military coup.

Mr. Nixon’s national security adviser, Henry Kissinger, worried that a Marxist such as Mr. Allende rising to power through democratic elections would become a model for other countries in the region, and also for Europe.

The Nixon administration deployed a variety of policies and covert operations to maximize pressure on the Chilean government. This included a C.I.A.-funded anti-government propaganda media campaign; blocking loans to Chile from multilateral financial institutions; offering secret funds to foment strikes; and assuring the Chilean military it would have full U.S. support should it move to oust Mr. Allende.

With the capital firmly under military control, Mr. Allende ordered everyone left in La Moneda to surrender. He stayed behind and shot himself minutes later.

For decades, many on the left in Chile and across Latin America claimed that the military had killed him, despite witness testimony and the fact that troops had not yet stormed the building.. A judicial investigation, which led to the exhumation of his remains in 2011, concluded that the president had indeed taken his own life.

With Congress closed, the legislature and the Constitution, in place since 1925, were replaced by a set of decrees and laws designed and approved by the military junta — including at least 150 secret laws, many of them authorizing the allocation of millions of dollars to the military.

No elections were allowed and all of Chilean society was militarized: Officers or pro-junta civilians would become unelected mayors, university chancellors, cabinet members and legislators.

A fraudulent election in 1980 approved an authoritarian constitution that gave General Pinochet another eight years in power and set the conditions for Chile’s transition back to a democracy. Today, a constitutional council controlled by the right is weighing a draft to replace the Constitution, and it will be submitted for a vote in December.

Over 200,000 Chileans were forced into exile, and one of the missions of DINA, the secret agency, was to infiltrate exiled communities and neutralize opposition leaders.

The agency organized a notorious program known as Operation Condor that coordinated intelligence services in South American countries under military rule to facilitate the exchange of information and prisoners and provide support for assassination plots against opponents in several continents.

In 1976, DINA, collaborating with other Condor countries, used a car bomb to kill Chile’s former ambassador to the United States, Orlando Letelier, in Washington. An American, Ronni Moffitt, also died as a result of the bombing.

Tens of thousands of men and women were rounded up during the first few months after the coup and held in massive detention centers throughout the country, including the National Stadium in the capital.

The junta called on people to turn in any suspicious foreigner, and thousands of them, mainly Latin American political refugees fleeing their own dictatorships, were detained.

About 1,200 clandestine detention and torture centers were set up across Chile, where more than 40,000 people were tortured, according to official records.

Civil and political liberties were suspended, political parties and labor unions were outlawed and a state of siege and strict curfew was imposed. Opposition media was banned or censored, dissent was severely punished, and books and other publications considered leftist or subversive were destroyed.

Universities’ liberal arts and social science departments were closed and left-leaning professors and students were expelled, detained or killed.

General Pinochet stepped down as the army’s commander in chief in 1998 and was arrested in London, pending an extradition request from Spain, where he was being investigated for the murder of Spanish citizens in Chile. Britain allowed his return to Chile for humanitarian reasons.

When he died in 2006 at 91, General Pinochet, his wife and his children were being investigated on fraud and corruption charges.

General Pinochet was indicted on charges in connection with high-profile human rights crimes, but was never convicted. Today, some 270 military officers and agents are serving sentences for human rights crimes.