A Few Thousand Rioters. A Few Dozen Police.

It was an unfair fight in front of Brazil’s Congress. On one side of a metal barrier were a few dozen police officers, some armed with pepper spray, others with clubs. On the other was a rapidly growing mob of more than 1,000 angry protesters, falsely convinced that the presidential election had been stolen and dead-set on doing something about it.

At 2:42 p.m. on Sunday, almost in unison, protesters at one end of the street easily pulled down the metal barrier, while at the other end, protesters pushed right through a plastic roadblock, according to a video obtained by The New York Times. A few police officers sprayed chemical agents, but within seconds, the crowd was surging through.

The moment was the start of a riot that left Brazil’s Congress, Supreme Court and presidential offices ransacked and the nation’s democracy under its worst threat in decades. And the previously unpublished video of the moment also lays bare the woefully inadequate security at some of the nation’s most important institutions, which is now at the center of the wider investigation into how the mayhem could have occurred, despite ample warning signs.

Federal authorities have laid much of the blame on the handful of men who run the federal district that includes Brazil’s capital, Brasília. They accuse the district’s governor and security chief of being either negligent or, worse, complicit, and they have already taken action against them.

In the hours after the riot, Alexandre de Moraes, a Supreme Court justice, suspended Ibaneis Rocha, the district’s governor, from his post for at least 90 days. Mr. Moraes then approved an arrest warrant from the federal police for the district’s security chief, Anderson Torres, as well as its police chief, Fabio Augusto Vieira. In votes on Wednesday, the Supreme Court confirmed both orders.

Mr. Moraes, a controversial figure who has been criticized for overstepping his authority, said evidence showed the men knew that protesters were planning violence, but did little to stop it.

Neither he nor other federal authorities have disclosed that specific evidence. Instead, he cited the inadequate number of security forces and the fact that roughly 100 buses of protesters were allowed to enter Brasília with little monitoring.

What is clear is that the federal government largely ceded responsibility to the district to protect the capital in the face of protests that, according to a slew of social media posts in the days prior, appeared likely to turn violent. The federal government pays the district roughly $2 billion a year to provide security, and the district had successfully protected the capital during several large, tense political events in recent months.

A four-page security plan obtained by The Times showed that, during the planned protests on Sunday, much of the responsibility for protecting the federal government’s buildings fell on the district police.

The document, which was signed Friday afternoon and sent to more than a dozen top security officials in Brasília, tasked the district police to keep demonstrators out of Three Powers Plaza, which includes Congress, the Supreme Court and the presidential offices, and to “maintain reinforcement of personnel” throughout the protests.

But that plan did not please Flávio Dino, Brazil’s justice minister, when he heard about it on Saturday morning in a phone call with Mr. Rocha, the district governor, according to an official in Mr. Dino’s office who spoke on the condition of anonymity because officials had not yet agreed to release the details of the call.

Mr. Dino did not want protesters on the national esplanade, Brazil’s version of the National Mall in Washington, a long grassy stretch that leads directly to Brazil’s most important government buildings. In response, Mr. Rocha agreed to change the plan accordingly and make the esplanade off limits, according to the official in Mr. Dino’s office.

Later that night, according to the official, Mr. Dino was surprised when he saw a news article that said Mr. Rocha would let the protest go forward on the esplanade with “tranquillity and security.”

The protests went forward, but the tranquillity and security was lacking.

On Sunday, thousands of supporters of Jair Bolsonaro, the ousted far-right president, marched onto the esplanade, dressed in the yellow and green of the Brazilian flag and carrying signs that demanded a military coup and that referenced voter-fraud conspiracy theories long peddled by Mr. Bolsonaro.

The district police was there, but not in full force. Authorities have not provided the precise number of police officers present on Sunday, but according to videos and eyewitness accounts, there were far fewer officers than for other recent demonstrations in the capital.

By contrast, there were several hundred thousand people in the same spot a week earlier for the inauguration of President Luiz Inácio Lula da Silva. While those crowds were there to celebrate, rather than wreak havoc, the district deployed all of its more than 10,000 police officers, far more than were on the ground on Sunday.

Why there were so few police officers is now a central question for investigators. The security plan did not list a number of officers, but instead just suggested that the police should have sufficient personnel to handle the protests.

Federal authorities have pointed the finger at Mr. Torres and Mr. Vieira, the district’s security chief and police chief, who have been ordered arrested.

Mr. Torres, in particular, has come under scrutiny. He was Mr. Bolsonaro’s former justice minister and started in his new post in the district on Jan. 2. He quickly replaced much of the district’s security staff, despite its recent track record of success during the elections, and then left for vacation in Florida, where Mr. Bolsonaro has also been staying in recent weeks.

On the day of the protests, Mr. Torres, who was ostensibly in charge of the capital’s security, was thousands of miles away.

Mr. Torres said Tuesday that he would return to Brazil to defend himself. “I have always guided my actions with ethics and legality. I believe in the Brazilian justice system and in the strength of the institutions. I am certain that the truth will prevail,” he said on Twitter.

Mr. Rocha, the district governor, has now also begun to point the finger at his deputies for the security lapses.

Alberto Toron, Mr. Rocha’s lawyer, said in an interview on Wednesday that the security plans were adequate, but that the security forces failed to carry them out, even suggesting that they did so deliberately.

“We saw videos, for example, of police fraternizing with demonstrators,” he said. “There is a hidden hand here, which not only demobilized the police and the Army not to act, but it seems that there was an orchestration for something broader to happen.”

“The governor was deceived,” he added. “He suffered a process of sabotage.”

Several videos appear to show the police as indifferent to the protests. In one, a man asks a group of chatting police officers if he can walk all the way to the end of the esplanade and take a bath in the reflecting pool in front of Congress. “Everything is open today?” he asks. The police appear to respond affirmatively, and wave him in the direction of Congress.

Another video shows that after protesters ascend onto the roof of Congress and break into the building, about 10 relaxed police officers watch the scene, chatting with protesters, texting and filming the scene themselves.

It was not until the protesters had broken inside the government buildings that military and federal law enforcement arrived to retake control.

Federal security officials in charge of protecting the presidential offices had not expected violence during the protests, and only asked for reinforcements from the Army after rioters broke inside the building, according to an Army general who spoke anonymously to discuss a sealed investigation.

Federal police said late Wednesday that they had arrested 1,159 people, nearly all under the suspicion of taking part in the riots. Authorities have said in recent days that they are now turning their attention to the political and business elites who helped organize, fund and aid the riots.

The actions of security officials and police officers are expected to remain a central focus of investigators in the months ahead. Brazil’s Senate plans to begin a congressional investigation next month. On Wednesday, 60 U.S. and Brazilian members of Congress released a joint statement, condemning extremism in both countries that led to attacks on their capitols.

Lis Moriconi and Leonardo Coelho contributed reporting.