It was not immediately clear which side was in control of the embassy. A Brazilian foreign ministry representative was inside the compound in an attempt to end the stand-off.
Guaido's appointed ambassador Teresa Belandria said in a statement that staff members at the embassy "voluntarily" opened its doors early Wednesday to officials working for her.
Hundreds of people gathered outside the embassy, many of them supporters of Brazil's left-wing Workers Party, yelling "Viva Maduro," "Drug dealer Guaido" and "Free Lula," the Brazilian leftist icon released from jail on Friday. Police detained two people after scuffles broke out between Guaido and Maduro supporters.
The last Maduro-appointed ambassador to Brazil was withdrawn in 2016.
In her statement, Belandria asked that all accredited officials at the embassy and Venezuela's seven consulates in Brazil recognize Guaido as the legitimate president.
More than 50 countries, including Brazil, recognized the Venezuelan opposition leader as acting president earlier this year after rejecting Maduro's re-election as fraudulent.
BRICS titans China and Russia, however, back Maduro.
"Upon entering the headquarters, we could verify that a group of officials was living in the official residence," said Belandria, who is not inside the embassy.