Divided Brazilians rally
Pro- and anti-impeachment protesters gathered in various parts of the capital to make their way to the grassy esplanade in front of the Brazilian Congress.
There a 2-metre (6.5-foot) high wall has been erected stretching for more than 1 km (0.6 of a mile) to separate both sides, a symbol of the stark political divide in one of the world's most unequal societies.
Buses carrying some of the thousands of police being deployed in the capital Brasilia were arriving and officers getting into position. Further protests are expected in Sao Paulo and Rio de Janeiro.
Polls suggest more than 60 percent of Brazil's 200 million people support impeaching Rousseff, whose inner circle has been tainted by a vast corruption scandal at state oil company Petrobras.
The Workers Party, however, still musters strong support among millions of working class Brazilians, who credit its welfare programmes with pulling their families out of poverty during the last decade.
In the car park of Brasilia's soccer stadium, some of these supporters waved red flags and set off fire crackers, as they prepared to march on Congress.
"There won't be a coup, there will be a fight," the crowd shouted, referring to Rousseff's view that the move to impeach her has no legal grounding and is a coup d'état.