She was addressing the nation on TV for the first time since senators voted overnight to suspend her for budgetary violations and put her on trial.
Ms Rousseff said her government was “undergoing sabotage”.
Vice-President Michel Temer will assume the presidency while Ms Rousseff’s trial takes place.
The trial may last up to 180 days, which would mean Ms Rousseff would be suspended during the Olympic Games in Rio de Janeiro, which start on 5 August.
Senators had voted to suspend her by 55 votes to 22 after an all-night session that lasted more than 20 hours.
Ms Rousseff is accused of illegally manipulating finances to hide a growing public deficit ahead of her re-election in 2014.
‘Fraudulent’
In her TV speech in front of supporters at the presidential palace, Mr Rousseff said that she may have made mistakes but had committed no crimes, adding: “I did not violate budgetary laws.”
She said: “What is at stake is respect for the ballot box, the sovereign will of the Brazilian people and the constitution.”
Branding the process “fraudulent”, she vowed to fight the charges against her and said she was confident she would be found innocent.
Ms Rousseff, 68, accused the opposition of leading the impeachment because they had vehemently opposed all the advances she and her predecessor, Luiz Inacio Lula da Silva, had made for the Brazilian poor and lower middle classes.
After her speech she left the presidential palace and shook hands with supporters lining the pathway.
In another speech outside she told supporters she could feel their “love and energy” on what she called a “tragic” day for the country.
Michel Temer is set to be sworn in later on Thursday and he is expected to give a speech and present some of his cabinet.
During the overnight debate, Senator Jose Serra, tipped to become the new foreign minister, said the process was “a bitter though necessary medicine”.
“Having the Rousseff government continue would be a bigger tragedy,” he said.
Brazil is suffering from its worst recession in 10 years, unemployment reached 9% in 2015 and inflation is at a 12-year high.
Source: BBC
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