An incident occurred shortly before Luiz Inacio Lula da Silva was sworn in as President of Brazil.
Police on Sunday arrested a man who tried to gain access to the esplanade in the capital Brasilia with an explosive device and a knife.
A military police representative told Reuters news agency.
300,000 people are expected to attend the celebration
Luiz Inácio Lula da Silva has been sworn in as the new President of Brazil.
The 77-year-old took his oath of office in Congress on Sunday.
He had previously driven through the capital, Brasília, in an open Rolls Royce with his wife Janja and the new Vice President Geraldo Alckmin and his wife.
Thousands of supporters cheered him on.
More than a dozen heads of state attended the inauguration, including Federal President Frank-Walter Steinmeier.
Around 300,000 people are expected on the Esplanade to celebrate the inauguration.
The transfer of office to the left-wing politician Lula took place under increased security precautions.
The reason is threats from supporters of his right-wing populist predecessor Jair Bolsonaro, who does not recognize Lula's victory in the runoff election at the end of October.
Lula faces big challenges
Lula ruled the largest country in Latin America from 2003 to 2010.
He is now the first democratically elected president in Brazil to serve a third term.
He defeated his predecessor Jair Bolsonaro in the runoff election in October.
Contrary to custom, the right-wing ex-military did not take part in the swearing-in ceremony.
He had already traveled to the USA with his family on Friday.
Brazil's new president faces major challenges.
After his predecessor deeply divided society and isolated the country, the new president wants to reconcile Brazil and bring it back onto the international stage.
Lula announced a resolute environmental protection and climate policy and measures against increasing hunger.
However, he is dealing with a congress in which supporters of President Bolsonaro, who has been voted out, make up the largest faction.