Thousands of Brazilians gathered outside Army barracks in Rio de Janeiro, Brasilia and other cities on Tuesday demanding the military intervene to prevent leftist president-elect Luiz Inácio Lula da Silva taking power next year.
"We want a better Brazil. We don't want Lula to take charge on January 1. We don't want a communist country," bank employee Lais Nunes, 30, told AFP in Rio.
Protesters draped in green and gold waved Brazilian flags and sang the national anthem on what was a bank holiday.
"There is various information that there was electoral fraud ... We can't accept that," added police officer Leandro de Oliveira, 38, who claimed the national electoral tribunal was responsible for the supposed fraud.
Supporters of outgoing far-right President Jair Bolsonaro have alleged fraud surrounding the electronic voting system that has been used since 1996.
Bolsonaro himself did likewise repeatedly, without providing any supporting evidence.
Brazil's defence ministry has, however, produced a report dismissing alleged inconsistencies in the electronic results, while international observers also validated the election result.
Lula, who was also president from 2003-10 and left with sky-high approval ratings, won the October 30 run-off with just under 51 percent of the vote compared to Bolsonaro's 49 percent.
In the capital Brasilia, thousands more gathered at the army's headquarters with some holding up banners such as "S.O.S. Armed Forces" and "Audit at the polls."
Security was stepped up in the capital and police restricted access to the area around the presidential palace, parliament and supreme court.
Similar protests took place straight after the second round election last month.
Since then, many people set up a camp outside the army headquarters in Sao Paulo, where there were also protests on Tuesday, as well as in Belo Horizonte.
Apart from a brief speech two days after his defeat, Bolsonaro has remained tight-lipped and a recluse, with his official diary left empty.
He has not only disappeared from public life but also from social media, where he used to be extremely active, even running the majority of his successful 2018 campaign online.
He is not attending the Group of 20 summit in Bali, Indonesia, where Brazil is being represented by its top diplomat Carlos Franca.