Supporters of Brazilian far-right former President Jair Bolsonaro who refuse to accept his electoral defeat have stormed the presidential palace, Congress, and the Supreme Court in the capital, Brasilia.

Videos on social media showed Bolsonaro supporters smashing windows and furniture in the National Congress and Supreme Court buildings on Sunday. They climbed onto the roof of the Congress building, where Brazil’s Senate and Chamber of Deputies conduct their legislative business, unfurling a banner that read “intervention” and an apparent appeal to Brazil’s military.

Images on TV channel Globo News also showed protesters roaming the presidential palace, many of them wearing green and yellow – the colours of the Brazilian flag, which have also come to symbolise the Bolsonaro government.

One social media video showed a crowd outside pulling a policeman from his horse and beating him to the ground.

Security forces used tear gas in an effort to push back the demonstrators with local media estimating about 3,000 people were involved in the incident.

The siege, which lasted a little over three hours, comes just a week after the inauguration of Bolsonaro’s leftist rival, President Luiz Inácio Lula da Silva.

Responding to the invasion, Lula declared a federal security intervention in Brasilia that will remain in place until the end of the month.

In a press conference, he blamed Bolsonaro and complained about a lack of security in the capital, saying authorities had allowed “fascists” and “fanatics” to wreak havoc.

“These vandals, who we could call fanatical Nazis, fanatical Stalinists … fanatical fascists, did what has never been done in the history of this country,” said Lula, who was on an official trip to Sao Paulo state. “All these people who did this will be found and they will be punished.”

Bolsonaro, who has yet to concede defeat in the October 30 vote and is currently in the US state of Florida, has peddled the false claim that Brazil’s electronic voting system was prone to fraud, helping to fuel protests against Lula’s win.

Supporters have blocked roads, set vehicles on fire and gathered outside military buildings calling on the armed forces to intervene.

“This genocidist … is encouraging this via social media from Miami,” Lula said, referring to Bolsonaro. “Everybody knows there are various speeches of the ex-president encouraging this.”

Bolsonaro was silent for nearly six hours about the chaos in Brasilia before posting on Twitter that he “repudiates” Lula’s accusations against him.

The former president added that while peaceful demonstrations were part of democracy invading and damaging public buildings “crosses the line.” He did not attend Lula’s inauguration.

Reporting from Rio de Janeiro, Al Jazeera’s Monica Yanakiew noted that some Bolsonaro supporters had been camped out in Brasilia since the election.

“People from this camp and from other parts marched toward the square in Brasilia, called the Three Powers square, because in this same square you have Congress, the presidential palace, and the Supreme Court and they’ve entered the three buildings,” she said.

“They went inside the Supreme Court, which they consider to be their main enemy, because they say that the Supreme Court is biased, and recognised an election that they say is stolen,” Yanakiew said, noting that the incident occurred after Lula’s January 1 inauguration, when authorities were less likely to expect such a siege.

However, she added the “big question” remained as to why demonstrators were so easily able to overrun security forces during the incident, which took place when legislators, justices and other officials were not on the premises.

The storming recalled the January 6 invasion of the United States Capitol by supporters of former President Donald Trump, who like Bolsonaro’s supporters, also claimed without evidence that the 2020 US presidential election was “stolen”.

US President Joe Biden described the situation in Brazil as “outrageous” and that he looked forward to continuing to work with Lula.

“I condemn the assault on democracy and on the peaceful transfer of power in Brazil. Brazil’s democratic institutions have our full support and the will of the Brazilian people must not be undermined,” Biden said on Twitter.

Brazilian law professor Thiago Amparo said that like Trump, Bolsonaro had for years fomented distrust in government institutions.

“It’s the kind of rhetoric that was seen not only in the election cycle, but throughout the whole presidency of Bolsonaro,” he told Al Jazeera. “So this moment is really the concrete symbol of several years of trying to discredit the political and judicial institutions in the country.”

He added that many local officials and members of the armed forces maintain ties with Bolsonaro, making it a “very complicated situation” for Lula and his government to navigate.

On Twitter, Brazil’s Justice Minister Flavio Dino said: “This absurd attempt to impose their will by force will not prevail.”

“The government of the Federal District has ensured there will be reinforcements. And the forces at our disposal are at work,” he said.

Chief Justice Rosa Weber and Justice Alexandre de Moraes pledged punishment for the “terrorists” who had attacked the country’s democratic institutions, while the heads of both houses of Congress denounced the attacks publicly.

Senate President Rodrigo Pacheco said he was in permanent contact with Brasilia’s Governor Ibaneis Rocha, and said that the entire police apparatus had been mobilised to control the situation.

Rocha later said on Twitter that more than 400 people had been arrested over the invasion. But the Brazilian police later revised the figure down to 300.

Rocha said those arrested “will pay for the crimes committed”.

“We continue working to identify all the others who participated in these terrorist acts this afternoon in the Federal District. We continue to work to restore order,” he tweeted.

Venho informar que mais de 400 pessoas já foram presas e pagarão pelos crimes cometidos. Continuamos trabalhando para identificar todas as outras que participaram desses atos terroristas na tarde de hoje no Distrito Federal. Seguimos trabalhando para que a ordem se restabeleça.

— Ibaneis Rocha (@IbaneisOficial) January 8, 2023