"There will be no coup," chanted protesters clad in red shirts as they marched through downtown Sao Paulo.
Chanting against the "coup" has historical significance, prompting memories of the South American country`s 1964-1985 military dictatorship, under which Rousseff was jailed for two years and tortured.
"I`m here to protest the political aberration that is seeking to topple a democratically elected president," pro-Rousseff activist and attorney Clarise De Almeida told the AFP news agency.
Demonstrations were staged across 26 of Brazil`s 27 states. Organizers said the marches amounted to more than 200,000 people, although police estimated crowds at a significantly fewer 83,000 people.
Just shy of a year into her second term, Rousseff has witnessed her approval ratings drop to 9 percent, a record low for the leftist politician. Rousseff has been struggling against an economic recession, fiscal deficit, inflation and increasing unemployment.
Impeachment ruling
The Brazilian Supreme Court is set to decide on the legality of a congressionally established commission seeking to impeach the president.
Controversy has surrounded the formation of the committee after allegations that a secret congressional vote was stacked with Rousseff`s opponents.
Judges of the Supreme Court is set to vote on Thursday. If approved, the Senate would have to open an impeachment. The move would likely lead to Rousseff`s suspension of up to six months.
Lower house speaker Eduardo Cunha, who initiated the impeachment process following a request from opposition lawmakers, had his home raided by police under a Supreme Court-ordered warrant that also included other politicians.
Cunha is accused of accepting bribes of up to $40 million (36.8 million euros) and stashing it in Swiss bank accounts to avoid detection from government authorities.
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