Brasilia rioters likely had inside help

Brazil intensified a mop-up operation after the shock weekend ransacking of government buildings in the capital, as President Luiz Inacio Lula da Silva said the rioters likely had inside help.

Lula told reporters he had ordered a "thorough review" of presidential palace staff after Sunday's violent uprising, which saw backers of his far-right predecessor Jair Bolsonaro storm the presidency, Congress and Supreme Court causing widespread damage.

"I am convinced that the door of the Planalto (presidential) palace was opened for people to enter because there are no broken doors," the president said in Brasilia. "This means that someone facilitated their entry," added Lula, who also pointed a finger at "conniving agents" from the police and armed forces.

Dealing with the aftermath of the violent reaction by so-called "bolsonaristas" to his brand-new presidential term, Lula added that "from now on we will be tougher, more cautious, more prudent."

He said any "radical 'bolsonarista'" found still working for the government will be dealt with, and cited media reports of alleged threats made by staffers inherited from the previous administration.

"How can I have a person outside my office who might shoot me?" asked the president, who narrowly defeated Bolsonaro in an October vote following a deeply divisive campaign.

Human Rights Watch, meanwhile, urged the government to "thoroughly investigate all those who have incited, financed, or committed acts of violence in an attempt to negate election results."

Pro-Bolsonaro rioters on Sunday looted government offices, destroyed priceless works of art and left graffiti messages calling for a military coup.